The purpose of this study is to investigate effect of STEM activities on the scientific process skills, scientific epistemological beliefs and science achievement of 8th grade students. The research was carried out with using a non-equivalent control group pretest-posttest quasi-experimental design, one of the quantitative research methods. In the pretest-posttest control group quasi-experimental design, one of the two groups is assigned as the control group and the other as the experimental group and the pre-test is applied before the experimental application (Büyüköztürk, Akgün, Demirel, Karadeniz & Çakmak, 2015). So the study, 18 students (11 female, 7 male) were in the experimental group and 14 students were in the control group. The study was carried out with the participation of 32 students who were selected in the 8th grade in a public school in Adıyaman selected with the convenience sampling method. In the experimental group, implementation was carried out with constructive learning approach supported by STEM activities, and in the control group with constructive learning approach. STEM Lesson Plans and STEM Rubrics prepared by adapting the teaching plans prepared by Çorlu and Çallı (2017) were used in the experimental group. In the control group, the courses were conducted in accordance with Science Curriculum which was prepared in accordance with constructivist learning approach (MEB, 2018). The study was carried out with 3 different units (Human Reproduction, Growth and Development, Simple Machines and Structure and Properties of Matter) and 4 different topics (DNA and Genetic Code, Leverage, Inclined Plane and Periodic System) for a total of 5 weeks. In order to collect data, “Science Process Skills Test” and “Scientific Epistemological Beliefs Scale”, and “Science Achievement Test“ were used. The difficulty indexes of the items included in the final test of the science achievement test developed by the researchers ranged from 0,41 to 0,72. In addition, item discrimination index of the questions included in the test is in the range of 0,31 to 0,58. The average difficulty of the test was 0,61 and the mean discrimination index was 0,48. The reliability coefficient of the test was 0,903. Therefore, it can be said that the science achievement test developed is a valid and reliable measurement tool.). The scientific process skills test, which was adapted to Turkish by Aydoğdu (2006), consisted of 0,81 reliability coefficient, 4 options and 25 items. The “Scientific Epistemological Beliefs Scale”, which was adapted to Turkish by Özbay (2016), and whose validity and reliability studies were carried out, is 5-likert type. The reliability coefficient of the scale consisting of 26 items was found to be 0,80, Since the data fit the normal distribution, it was decided to perform parametric tests. The study was conducted with 34 students and a school in rural areas. The effect of STEM activities can be investigated by conducting research with a large sample and in a school of different socio-economic level. Significance level criterion was taken as 0,05 when interpreting the results. In addition, the effect size values of the statistically significant results were calculated (Cohen, 1988). Cohen (1988)'s criteria were taken into consideration when interpreting effect size values (d ≤ 0,2 means small, 0,2 <d <0,8 means medium and d ≥ 0,8 means large effect size).In the first sub-problem of the research, the effect of STEM activities on the scientific process skills of 8th grade students was examined. According to the results, it was determined that there was a statistically significant difference between the pre-test and post-test scores of the experimental group in terms of science process skills, while there was no significant difference between the pre-test and post-test scores of the control group. In addition, no statistically significant difference was found between the experimental and control groups' science process skill post-test scores. In the second subproblem of the research, the effects of STEM activities on the scientific epistemological beliefs of 8th grade students were examined. In terms of scientific epistemological belief, there was no statistically significant difference between the pre-test and post-test scores of the experimental group and the control group. However, there was a