This research mainly aimed to understand ninth-grade students’ basic comprehension of Circumcenter, Incenter and Triangle Centroid and how they solved problems, analyze the error patterns in their performance of Circumcenter, Incenter and Triangle Centroid, investigate the possible reasons of their errors. The research method combined surveys and interviews. The sample included twenty eight ninth-grade students from a class in a junior high school in East District, Tainan City. Applying “achievement test”, a paper-based test developed by the researcher, to the Circumcenter, Incenter and Triangle Centroid session, this research investigated students’ performance in the achievement test and the errors in their performance. The error patterns were generalized from the result of the achievement test. Ultimately, the researcher conducted interviews with students to further understand the possible reasons for the errors they made in the achievement test. The main results were: first, the overall error rate was over fifty percent. The highest error rate occurred in Circumcenter, followed by that of Incenter, with that of Triangle Centroid the lowest. Next, there were seven error patterns in students’ performance of Circumcenter, Incenter and Triangle Centroid. In the end, there were five possible reasons for students’ errors in their performance of Circumcenter, Incenter and Triangle Centroid.