The objective of this study is to determining the change in secondary school students' cognitive structure related to the concepts of environmental problems. There were 21th grades students of two middle schools in Kirsehir. In this research, the students participated in outdoor activities on environmental education programmes during 5 weekends consecutively. The data of the research were collected via word association test as pre-test and post-test. The conceptual change process was examined in collected data, and their pre-test and post-test results were analysed by using cut-off point. Conceptual network was constituted by using cut-off points. When pre-test and post-test results were compared, it was found that there is a positive shift in students' conceptual change. In addition, when their concept network was examined, it was observed that there is an upturn in the number of answered words in post-test. However, the numbers of the words associated to the keywords in pre-test were limited. Besides, the connections not provided among the concepts in pre-test emerged in post-test which shows that students' independent knowledge related to the keywords is associated in the post-test. Finally, when pre-test and post-test are compared, both are the increased in response words and emergence of the connection shows that qualitative change accompanies to quantitative change. Furthermore, keywords emerged in upper tabs out of post-test results. This specifies that the out-of school activities is successful in creating conceptual structure.