Objective To study the retentive force and deformation of acetal resin clasp. Methods 40 premolars and 40 molars were cast respectively. Undercut of 0.25 mm or 0.50 mm depth were measured for each with undercut gage. According to the type of abutment and the depth of undercut, the specimens were divided into 4 groups: Premolars with 0.25 mm undercut, premolars with 0.50 mm undercut, molars with 0.25 mm undercut and molars with 0.50 mm undercut, 20 specimens each group. 10 three-arm clasps with resin and Co-Cr alloy were fabricated in each group, respectively. The clasps were set into the corresponding abutments and soaked in distilled water. The retentive force of the clasps when 0, 720, 1 440, 2 160, 2 880, 3 600, 4 320 consecutive times of setting in and removing out from the abutments were measured. The distance between the tips of retentive arm and resistant arm after 0 and 4 320 cycles were recorded. Results 1）The mean retentive force of resin clasps（1.69 N） was significantly lower than that of Co -Cr clasps（5.87 N）（P
