As an action research project, using mixed methodology, this study investigated how the use of math journals affected second grade students' communication of mathematical thinking. For this study, math journal instruction was provided. The data gathering included pre- and post- math assessment, students' math journals, interviews with the students, and teacher's reflective journal. Findings of the study indicated that the use of math journals positively influenced the students' communication of mathematical thinking and the use of math vocabulary. Additionally, math journals served as a communication tool between the students and teacher and an assessment tool for the teacher. The implications of this study regarding students' writing ability and time constraints issues were also discussed.