The aim of this study is to investigate the effects of mathematics (math) diaries on academic achievement, metacognitive awareness and motivation of students in a mathematics lesson. The case study was conducted with a qualitative, quasi-experimental design and a quantitative, embedded multiple-case design as a mixed research model. The sample of the quantitative part of the study consisted of 32 4th-grade students, 16 of whom were in the experimental group and 16 in the control group. The participants in the qualitative part of the study were 16 students from the experimental group where math diaries were used. The quantitative data collection instruments used in the study were the Perimeter Measurement Achievement Test for primary school students, the Mathematics Motivation Scale for primary school students and the Metacognitive Awareness Scale. The students' structured math diaries were used to collect qualitative data. The quantitative data obtained were analyzed using a One-Way ANCOVA and Paired Samples t-test, while the qualitative data from the math diaries were thoroughly analyzed using a qualitative content analysis. As a result of the one-way analysis of covariance, statistically significant differences were found between the adjusted posttest means of the groups, which were adjusted for pretest scores on student achievement, metacognitive awareness, and motivation. In the qualitative dimension of the study, positive improvements in students' achievement, metacognitive awareness, and motivation were observed from the beginning to the end of the implementation process, as revealed by the analysis of their math diaries.