The aim of the present research was to examine the contribution of expository and journal writing activities to the cognitive development of 7th-grade students with varying levels of academic achievement in mathematics education. The qualitative research approach has been adopted in this research. The lessons were conducted in one 7th grade class of 28 students using a total of 26 expository writing activities and 6 journal writings designed to address the topics of "lines, angles and measuring angles, operations with integers, rational numbers, algebraic expressions, equations, ratio and proportion.‖ During the 14 weeks of lessons, three themes were created and various codes were established under these themes by means of content analysis of the written explanations obtained from students' expository writing activities and diaries. In addition, the expository writing activities were analyzed using an analytical scoring rubric (ASR). The findings were coded under three themes: (1) features of the students' explanations (2) use of mathematical language and (3) mathematical algorithms and calculations. According to the data obtained, it has been determined that writing activities contribute to the cognitive development of particularly those students with a moderate level of academic achievement. In addition, it was revealed that the responses provided in writing activities by students with different levels of academic achievement varied. Based on the results of the study, it is recommended that writing activities be implemented over a long and continuous period of time.