In order to clarify how fifth graders of different achievement level behave in math classes, two observations were conducted. In Study 1, one hundred and sixty children from sixteen classes were observed by time sampling method. The behaviors were categorized into individual actions, interactions with their teacher and interactions with peers. As results, the most frequent behaviors observed were “self-working” and “listening to a teacher” as categorized into individual actions. Although there was no difference in the frequency of task-related behaviors by achievement level, there were differences in the quality of behaviors by achievement level and sex. For example, high achievers engaged more in active learning, and boys engaged more in active learning and task-unrelated behaviors. In Study 2, the observation schema was revised so that it became more sensible to the relation between children's behaviors and teaching situations. The results were similar to that of Study 1, but low achivers showed more task-unrelated and passive behaviors to teachers when they engaged in the same task individually. The way teachers interact with high and low achievers were discussed, and some suggestions were provided.