3D design and modeling is an important part of design education. Self-directed learning (SDL) has been considered in order to effectively learn and teach 3D design and modeling. Exploring the relationship between students' SDL ability and their behavior in learning 3D design will not only promote the 3D design learning process and outcomes, but also enhance their SDL ability. This study explored the relationship among students' SDL ability, their learning behavior, and 3D works in a self-directed 3D design class. Twenty-two masters students participated in the study. Results showed that students had a relatively high SDL ability. Their SDL ability slightly increased with no statistical significance after the class. There were significant differences in 3D design and modeling learning behaviors between high-level SDL students and low-level SDL students. Students' SDL ability positively correlated with the quality of 3D works in terms of the feasibility, functionality, and creativity. Students had positive perceptions on the self-directed 3D design class in terms of both SDL experience and 3D design experience.