This study established an online game-based assessment to explore third and fourth graders' performance of computational estimation in addition and subtraction. Students' computational estimation strategies could be automatically recorded and classified in this assessment, and the Higher-Order DINA model was used as a measurement model in order to provide more information of individual learning status. A total of 729 students in Taiwan participated in this study. 349 third graders and 380 fourth graders randomly selected from two elementary schools. Results showed that (a) successful problem-solvers tended to perform computational estimation with good intuitive number sense or relied on item content to adopt effective computational estimation strategies, (b) fourth graders' estimation ability in addition and subtraction was significantly better than third-graders' ability, and (c) students at different ability levels used different computational estimation strategies and mastered different computational estimation skills.