Problem-solving categorization tasks have been well studied and used as an effective tool for assessment of student knowledge structure. In this study, a traditional free-response categorization test has been modified into a multiple-choice format, and the effectiveness of this new assessment is evaluated. Through randomized testing with Chinese college students, the multiple-choice categorization test has been shown to provide equivalent measurement compared to the open-ended test. In addition, the influence of including diagrams in problems on students' categorization performances has also been studied, which suggests that the inclusion of diagrams in problems can improve students' performances in identifying the relevant concepts. The results also show that the knowledge structures of the Chinese college students are in the early transitional stage between novices and experts.