The purpose of this study is to investigate the efficiency of learning plan implementation prepared with the cooperative learning method. In particular, the study addresses the effect of cooperative learning on students' achievement and their views regarding the "Systems in Our Body" unit of the 6th grade Science and Technology lesson. For this purpose, mixed method was used. The study is conducted in the second term of the 2013-2014 academic year, on a study group consisted of 7 girls and 13 boys, a total of 20 students of a private middle school in Istanbul. An achievement scale was utilized for the quantitative data and focus group interviews were hold for the qualitative data. While t-test was used for the quantitative findings, content analysis technique was used for the qualitative data. The result of the study indicated that CL method had a favorable effect on learning. The cooperation based learning-teaching environment provided cooperation, supported permanent learning, provided opportunities to be successful, contributed to the development of social and personal skills, but also caused worry as it requires students to be successful at all stages.