The aim of this study is to determine the effects of a Process-Based Writing Modular Instructional Program (PBWMIP), supported with graphic organizers and formative assessment, on second-grade primary school students' writing compositions and attitudes toward writing. This quasi-experimental research design study was conducted with 40 second graders who were chosen for either the experimental group or the control group. Data was gathered through written texts and attitude scales related to writing. Pretest and posttest scores of the experimental and control group were analyzed with a two-factor ANOVA test. The results indicate that while there was a statistically significant difference between the groups in terms of the quality of writing compositions in favor of experimental group, there was not a significant difference between the groups in terms of the students' attitudes toward writing.