Background. The debate on peer bullying among students is increasing rapidly in the last decades (Barrio & at al., 2008; Catherine & at al., 2007; Kartal & Bilgin, 2009; Kepeneckçi & Çınkır, 2006; Perren & Hornung, 2005). In fact, studies show that almost 15% of the students were bullied in schools (Olweus, 1994; Nansel & at al., 2001). Studies also found a strong relationship between bullying and psychological problems including anxiety, depression and sleep disorders etc. (Fleming & Jocobsen, 2009; Fitzpatrick & at al., 2007; Iverson, 2005; Kiriakidis, 2008; Menesini & at al., 2009; Skapinakis, 2011). However, a new form of bullying has emerged with the rapid growth in technology called cyber-bullying as a new kind of aggression in cyberspaces.Cyber-bullying is defined as &#8220;an aggressive, intentional act carried out by a group or individual, using electronic forms of contact, repeatedly and over time against a victim who cannot easily defend him or herself&#8221; (Smith & at al., 2008, p. 376). Studies on the frequency of cyberbullying indicated that cyber-bullying is spreading among high school students (Hoff & Mitchell, 2009; Ybarra & Mitchell, 2004a). As a result of this trend, huge body of literature on cyber-bullying came into exist (Dempsey & Storch, 2009; Hinduja & Patchin, 2007; Hoff & Mitchell, 2009; Patchin & Hinduja, 2006; Smith & at al., 2006; Smith & at al., 2008; Willard, 2007; Ybarra & Mitchell, 2004a; Ybarra & Mitchell, 2004b).However, we do not know the prevalence of the cyber-bullying among Turkish high school students yet, although few studies deal with the different aspects of the cyber-bullying (Arıcak, 2009; Baker & Kavşuş, 2007; Dilmaç, 2009). Purpose. The purpose of the present study was to investigate the prevalence of cyber-bullying among Turkish high school students. In addition, it was tried to explore the cyberspaces where the students become victims and bully. And finally, the effects of several demographic variables including gender, age, grade and internet using time on cyber-bullying were investigated.Method. 336 students, who were selected randomly from three high schools in Ankara and İstanbul in 2011, participated in the study. Data were collected through the &#8216;Cyber-bullying and Online Aggression Survey Instrument-CBOASI&#8217;, which was developed by Hinduja and Patchin (2009). The CBOASI was adapted to Turkish culture in the present study. During the adaptation studies, it was seen that the Turkish version of the instrument is valid and reliable. CBOASI contains two different sub-scales. The validity studies of sub-scales of the instrument were done with exploratory factor analysis and seen that both sub-scales had one-single factor.The reliability analysis produced a Cronbach Alpha Coefficient of .78 for the first sub-scale and .94 for the second sub-scale.The data were analyzed with frequency, percent, chi-square, t-test, one-way analysis of variance (ANOVA) and LSD test. SPSS 13.00 was used for data analysis. Findings. The findings of the study showed that 14 percent of the participants became victim during the last one month on different cyberspaces. However, 10 percent of the participants were found to be bully. It is also seen that the most frequently used cyberspace where the cyber-bullying take place is social sharing web sites called &#8216;Facebook&#8217; (f=65) Mobile phones are also the mostly used electronic devices on which the cyber-bullying occurs (f=55). The findings also revealed that gender has not any significant effect on cyber-bullying [t(334)=1.32; p>.05].Likewise, age [F(3&#8211;332)=1.00; p>.05] and grade [F(2&#8211;333)=1.14; p>.05] do not produce any difference. However, participants who use internet more than five hours in a day relatively become bully as compared to other groups [F(4&#8211; 331)=2.95; p<.05]. Discussion, Conclusion and Implication. The findings of the present study are consistent with the previous studies (e.g. Hoff & Mitchell, 2009; Nansel & at al., 2001; Olweus, 1994; Ybarra & Mitchell, 2004a). In those studies the prevalence of the cyber-bullying among students was found to be relatively high. As it can be seen in the related section, present study has also revealed similar result. This result imply that cyber-bullying among Turkish high school students is as common as in other countries.With the current study it is also understood that &#8216;Facebook&#8217;, which is the most preferable social sharing web site on the internet, is also the platform where the cyber-bullying mostly takes place. Based on this result it can be said that education programs about the cyber-crimes on the internet to the students might prevent them from the cyber-bullying. The present study also implies that there are not any significant relations between cyber-bullying and gender, age and grade. And lastly, frequent internet users are relatively more cyber-bully.