The purpose of the study is to explore the effect of writing to learn activities including letter, summary, and poster on academic achievement in the fourth grade science course called introduction to matter unit. The study was carried out via pre-test post-test control group design, one of the quasi experimental research designs. The achievement test developed by the researchers about the unit of introduction to matter was used as data collection tool in the study. The achievement test about the unit of introduction to matter was administered to three experimental groups and one control group chosen randomly among the 4th grades as a pre-test. Then, the first experimental group students were asked to write a letter, the second experimental group was asked to write a summary and the third group was asked to prepare a poster as a writing to learn activity for each part of the unit. The control group students solved questions about the subjects learnt in this process. At the end of the implementation, the achievement test was administered as a post-test and then as a retention test three weeks later to all groups. The paired sample t-test (or the dependent ttest for paired samples) and one-way analysis of variance (one-way ANOVA) were used for data analysis. The findings of the study reveal that the fourth graders' post-test and retention test results for the introduction to matter unit in science course is statistically in favour of the experimental group students. As a result of the study, it was found that the academic achievement of the students who used writing to learn activities was higher and what they have learnt was more permanent than the students who learned the unit with current methods.