This research aims to investigate the effect of Montessori method on social competence and behaviors of 3.5-5 years-old-children on their emotion regulation skills. Sequential Explanatory Design, one of the mixed method designs, was used in the study. The study group of the research consisted of 55 children attending two independent preschools in Eskisehir. Personal Information Form, Social Competence and Behavior Evaluation Inventory-Teacher and Parent Forms, Emotion Regulation Checklist and Parent Interview Forms for the Evaluation of Montessori Method have been used to collect the data. Friedman test used for data analysis. Post-hoc analysis with Wilcoxon signed-rank test and Mann-Whitney U were conducted to reveal the source of differentiation between the scores. It was determined that significant differences between Social Competence - Behavior and Emotion Regulation Skills sub-scale pretest and posttest mean scores of children in the study group. There are significant differences between the posttest scores of study and control groups.