Pretwisting the patellar tendon graft has been advocated hy many authors. Lambert has described a 90 degrees pretwist to benifit the fat pad vascular supply as well as to "simulate the helicoid course of the ACL". This technique has also been recommended to fascilitate fixation. Jackson has recommended a 90 pretwist to decrease the possibility of abrasive wear on the tunnel edge, and also to fascilitate fixation as much as a 180 degrees pretwist has heen recommended. However, in spite of all these recommendations, little work has been done on the effect of such pretwisting on the biomechanical behavior of patellar tendon grafts. The purpose of this study is to discover, by experiment, the effects of a 360 degrees external rotation pretwist on the following tensile mechanical properties of canine central 1/3 patellapatellar tendon-tibia complex : (a) ultimate stress, (b) ultimate strain, (c) average elastic modulus, and (d) strain energy density. Five groups of each S specimens were divided as group 1 (0 degrees external rotation), group 2 (90 degrees external rotation). group 3 (180 degrees external rotation), group 4 (270 degreesexternal rotation), and group 5 (360 degrees external rotation). The direction of pretwist meant the twist of tibia around patella. The effects of pretwist were analyzed using parameter one-way analysis of variance (ANOVA)(P1) and nonparameter Kruskal-Wallis one-way analysis of variance (ANOVA)(P2) with repeated measures followed by Scheff multiple comparison test. A p values of < 0.05 was considered statistically significant. Among each groups ultimate stress (P1=0.059, P2=0.095), ultimate strain(P1=0.154, P2=0. l65), and average elastic modulus(P1=0.814, P2=0.818) were not statistically significant. Only strain energy density was the statistically significant difference between group 2 and group 5(P1=0.008, P2=0.022). As a result, there were observed that the ultimate stress and strain energy density were decreased and ultimate strain was decreased but there were not statistically significant differences hetween groups because of the small number of samples, and there will be advantages for graft to avoid ahrasive wear on the tunnel edge and lateral femoral condyle. But, more studies are needed for human being in the future.