Mathematical reasoning structure in solving problems through the following four steps, namely problematic situation, strategy choice, strategy is implemented, conclusion. At first glance it looks the same as the problem solving steps. The difference is, A (sub) task is met, which is denoted problematic situation, if it is not obvious how to proceed. Students are able to choose strategies through predictive arguments, ranging from procedural to strategies built by students themselves. Students are also able to implement strategies that students think are right to solve problems through verification arguments in order to get a conclusion. The reasoning structure is important for students because it requires students not only to solve problems and get the right results, but to pay more attention to how the students do the process and the reasons students choose and implement strategies to solve problems so that students can believe the truth of the conclusions students give. Nonetheless, students still tend to focus more on the final result or solution to a given problem. Students cannot give reasons that the solution given is correct. This happens because someone with a low working memory capacity experiences obstacles in choosing and associating the right information and is needed in solving problems so that the ability to determine strategies and implement strategies to solve problems is very lacking. Working memory plays a role in remembering, sorting and processing information so that it is useful to solve problems, so that the capacity of working memory becomes very important in the structure of mathematical reasoning. Based on this background, this study aims to determine the Mathematical Reasoning Structure of Middle School Students in solving Problems Reviewed Based on Working Memory Capacity. This research is a qualitative research. The study was carried out at SMPN 1 Makale. The selection of research subjects was carried out by providing a test to measure students' working memory capacity. The test used is the O-SPAN Test and selects 1 student with a high category working memory capacity and 1 student with a low category. Then give a test to investigate the structure of reasoning. The mathematical reasoning structure of research subjects with low working memory capacity indicates a problematic situation, strategy choice, conclusion but does not indicate strategy is implemented. Subjects can provide conclusions but cannot provide verification arguments that the strategies used are correct or reasonable and can solve the problem. And the mathematical reasoning structure of students with research subjects with high working memory capacity fulfills the four mathematical reasoning structures namely problematic situation, strategy choice, strategy is implemented, conclusion.