The aim of this study was to investigate the effect of project-based learning environments on 4th class students' conceptual understanding of recycling. The case study method was used in this study. The effects of project-supported learning environments on students' conceptual understanding were investigated. The sample group of the study consisted of 18 [10 girls and 8 boys, mean age: 10.78] grade 4 students in a primary school in Trabzon, Turkey. One voluntary teacher from the same school participated in the research. A conceptual understanding test including 2 open ended questions and semi structured interviews was used in the data collection process. Interviews were conducted to investigate the effectiveness of project-based learning environments on the conceptual understanding of students. Semi structured interviews including two questions were carried out. Students were divided into 4 groups at the beginning of the study. Each group developed its own project related to "recycling". The semi structured interview was conducted with 8 voluntary students. The students' conceptual understanding was presented in detail with semi structured interviews. It was seen that seven students' described "recycling" associating with the "papers missing" in the pre-interview. At the end of the study; it was found that the project-based learning environment had a positive effect on 4th grade students' conceptual understanding of recycling (t= -8,695, p<0.05).