Body mass index and physical flexibility are the important norms of physical fitness. Therefore, the detailed exploration on the relation between body mass index and physical flexibility is needed. The study analyzes and explores the resource of freshman students' body mass index and physical flexibility (bending forward in sitting position) in a university in southern Taiwan from 95th school year to 97th school year in order to understand the relation between the insufficiency, moderation and overweight of students' body mass index and physical flexibility (bending forward in sitting position). The result shows the group with moderate body mass index generally do good job. Except the male students of the 96th school year, the physical flexibility of students with overweight body mass index is better than students with insufficient body mass index. That is different from the books and records related with physical fitness which show that fat piling around the action joint will affect the joint and low down the physical flexibility. The result will be studied more deeply.