Predicting the actual performance of strategy use with self-report questionnaires is difficult, especially among elementary school children. Nevertheless, due to the simplicity of self-report questionnaires, it is desirable to identify one that can predict children's performance of actual strategy use. This study investigated whether a self-report questionnaire on the frequency of using a diagram strategy in mathematical problem-solving can predict children's actual diagram use by manipulating the question type (i.e., free description, multiple-choice, and Likert scale). We also examine the question types that can better predict the actual strategy use in elementary school children. Fourth- to sixth-grade children were asked to complete both a questionnaire, which asked about their daily use of a diagram strategy through three question types and a test consisting of math word problems. We found that when children were asked to self-report their strategy using a Likert scale, they were predicted to use diagrams during the test regardless of grades. Furthermore, the older the children became, the more effective it was to ask them to self-report in a free description type. These results suggest that appropriate question types can make it possible to measure actual strategy-use behaviors through self-report measures, even for elementary school children.