Aim:In this study, it was aimed to determine mothers’ Rational Use of Medicines (RUM) having children under five years old, and factors affecting theirmedication use.Method:This descriptive and cross-sectional study was performed in Pinarhisar, a districtofKırklareli, a provincein northwestern Turkey,between June 2015 and March 2016.The study sample consisted of 320 women having 0-59-months old children enrolled in Family Health Centers.Thedata were collected through face-to-face interviews using theQuestionnaire developed by the researchers.Results: The rate of RUM among the participantswas 30.0%. The rate of taking over-the-counter medication, and antibiotics without a prescription among the women was 90.0% and 19.1% respectively.The rate of thewomen who gave over-the-counter medication, and antibiotics without a prescription to their children was 66.3% and 6.3% respectively. Ofthe participants, 38.4% had medication at home to use just in case and 33.4% took medication upon others’ advice. Therates of correct responses given bythe mothers to questions on antibiotic use ranged between 18.8% and 52.2%. In the multivariate logistic regression analysis,the rate of rational use ofmedicines was higher in those who had a nuclear family,those whoseincome was above the minimum wage (1400-2800 Turkish liras), those who wereaware that the effectiveness of the drug would not increase as the drug price increased, and those who were aware of the rational use of drugs than it was inthose whose income was below the minimum wage (<1400 Turkish liras), those who had a large or fragmented family, those who were not aware that theeffectiveness of the drug would not increase as the drug price increased, and those who were not aware of the rational use ofdrugs respectively.Conclusion:About one out of three mothers who had children under the age of five used drugs rationally. Among the mothers, the rate of t aking over-the-countermedication and antibiotics without a prescription for themselves was higher thanthe rate of giving them to their children.Among the determinants ofrational use of medication were the family type, income level, and the level of knowledge or awareness of RUM. When mothers having children under theage of five present to health institutions, the interview processes should be evaluated well, educational studies should be planned for them and theirparticipation in these studies should be ensured.