The purpose of this study is to find out the cognitive strategy during the mental rotation activities and to find out the changes of spatial sense accordingly. To do this, this study developed a mental rotation activity program for six sixth-grade students and conducted five mental rotation activities twice each, a total of 10 times. As a result, first, cognitive strategies vary depending on the complexity of mental rotation activities. Second, simultaneous use of both overall and partial approach strategies and psychological and analytical processing strategies, perspective fixation, and perspective change strategies can increase the accuracy of recognition. Third, there is a tendency to use cognitive strategies differently in terms of processing methods, depending on the task of mental rotation activities. Fourth, the ability to rotate the sense of space has been improved by the mental rotation activity. As a result of this study, the following conclusions could be obtained. Cognitive strategies are used differently depending on the complexity of mental rotation activities, so students need to provide psychological rotation activities with varying degrees of difficulty to experience various cognitive strategies.