The aim of this study is to analyze the effects of project-based learning on students' academic achievement, attitude, and retention of knowledge in relation to the subject of "Electricity in Our Lives" in a fourth-grade science course. The study was conducted in a quasi-experimental design as a "pre-test, post-test with control group." In the experimental group, the unit was taught through the project-based learning method. The measuring tools were administered to both groups before and after the applications. To perfectly analyze the "process" of the method, seven different learning assessment "forms" were administered to the students. The findings of the forms indicated that the students learn to construct their own learning and to evaluate changes in their own behavior through the application of the method. The application of different methods between both groups had a statistically significant effect in terms of academic achievement, (F(1,112)?=?46.78, p?=?0.000) and of retention of knowledge (F(1,112)?=?35.24, p?= 0?.000). However, there were no statistically significant effects from being in different groups for the attitudes of students (F(1,112)?=?0.99, p?=?0.321). For the students, being in the project-based learning groups resulted in better academic achievement and retention of knowledge than being in the traditional teaching group.