In this research, the effect of using learning logs in the learning of length subjects of primary school fourth graders was investigated. The study group was composed of 35 state primary school students, determined by an easily accessible sampling method in the province of Tokat in 2014-2015 academic years. The method of the study is an experimental model with pre-test and post-test control group. In this study, "Learning Logs" which was developed by the researchers for the 4th grade mathematics course length measuring understanding test (LMUT) as the pre-test and final test data collection tool and learning logs prepared to create the effect on the pre-test-post-test data were used. LMUT was prepared as consisting of fifteen questions with four multiple choices after a meticulous preparation period with literature review. The experimental study was performed at the eight-week period. Data collected from pre-test and post-test were analyzed by covariance analysis and t-test. As a result of this study, it was seen that learning logs effected learning positively and the academic success of the course was positively boosted. It was also found that learning journals significantly increased the success of girl students compared to boy students.