Various teaching methods and techniques have been used for years to improve teaching English as a foreign language in Turkey. In order to increase student success, new methods and techniques are taking the place of teacher-centered traditional methods day by day. The cooperative learning method, which allows students to collaborate to achieve common learning goals rather than individual learning, has also become popular in English language classes. The purpose of this experimental study was to explore the effects of cooperative learning on EFL high school learners' reading comprehension. A pretest-posttest group research design was used. There was a total of 169 students involved in the study. The experimental groups (n=85) were treated with the techniques of the Student- Teams- Achievement Divisions (STAD) of the cooperative learning method for 4 weeks whereas the control groups (n= 84) were taught by the traditional method of direct instruction. An achievement posttest within the content taught was given to the groups at the end of the instruction. The mean scores of both groups on the test were compared through an independent samples t-test. The statistical results revealed the experimental groups progressed better than the control groups, indicating that the cooperative learning method was more effective on Turkish EFL learners' success of reading comprehension than the traditional method.