People constantly practice the act of thinking. They may not think about everything, yet they certainly think of something and make decisions. Critical thinking is a sort of reflective thinking. It is controlled and aimed at perfect thinking. It enables the self-realization of individuals in social life. Therefore, it is important to develop critical thinking at school stages and early ages. The purpose of this research was to investigate "the effect of foreign language learning activities on critical thinking skills of 4th-grade students". To achieve this purpose, researchers tried to answer these questions: (1) What is the effect of critical thinking activities on students' critical thinking? (2) What is the effect of critical thinking activities on students' personal definitions of critical thinking and critical thinker? (3) What is the effect of critical thinking activities on students' social skills? The method of this research is action research which is considered highly important for educational sciences. In the course of this research, activities for developing critical thinking skills were implemented to 4th grade students. 25 4th grade private elementary school students participated in the research. Researchers used 13-question "Critical Thinking Skill Open-ended Questionnaire" to collect data. The answers given to the open-ended questionnaire before and after the activities were analyzed using content analysis, and themes and codes were determined accordingly, thereby answers of the students are compared. It is noted that before implementation, most of the students did not have any idea about critical thinking or they thought critical thinking has a negative connotation; however, after implementation, they gained more positive and open-minded opinions about this term.