The aim of this study was to determine teachers' and students?' opinions on the effectiveness of cooperative learning. This study was conducted in Istanbul Lycee and it had a qualitative research design. The participant of the study assigned randomly among ninth grade students who were studying in Istanbul Lycee in the year of 2012 to 2013. Among 180 ninth grade students the researchers chose 15 male and 15 female students randomly (n = 30). Thus, the sample of this study consisted of 30 randomly assigned students. The researchers then chose 8 students randomly for the focus group interview and two geography teachers with 20 years of experience for the individual interview. The data were collected by means of a questionnaire form which consist of two open ended and ten semi-structured questions. Ten hour lesson plan developed by using cooperative learning method in the ninth grade geography lesson and applied two week period. In this study qualitative data analysis which consisted of classification of data, determining major themes used. The findings demonstrated that cooperative learning method was effective and increased student?s achievement.