This study is aimed at analyzing the level change and features of mathematical communication in elementary students’ expository writing. 20 students of 5th graders of elementary school in Seoul were given expository writing activity for 14 lessons and their worksheets was analyzed through four categories; the accuracy of the mathematical language, logicality of process and results, specificity of content, achieving the reader-oriented. This study reached the following results. First, The level of expository writing about concepts and principles was gradually improved. But the level of expository writing about problem solving process is not same. Middle class level was lower than early class, and showed a high variation in end class again. Second, features of mathematical communication in expository writing were solidity of knowledge through a mathematical language, elaboration of logic based on the writing, value of the thinking process to reach a result, the clarification of the content to deliver himself and the reader. Therefore, this study has obtained the conclusion that expository writing is worth keeping the students' thinking process and can improve the mathematical communication skills.