Tendon is a specialized connective tissue, which transmits the force generated from muscle contraction to the skeleton, thus making joint motion possible. Although injuries of tendon and their treatments are common practice in the field of orthopedic surgery, many problems are still in unsolved state, and in particular, the effect of early intermittent passive mobilization on each healing stage of tendon have been not clarified. To elucidate the effect of early intermittent passive mobilization on the biomechanical properties of healing tendon, flexor digitorum superficialis tendons of right pelvic appendages of 48 New Zealand white rabbits were cut and repaired, and were equally divided into 6 groups based upon the mobilization was not performed. The passive mobilization was comprized of a simple passive exten-2nd week, the 3rd week, the 4th week and the 5th week periods. In remaining eight rabbits, joint mobilization was not performed. The passived mobilization was comprized of a simple passive extension of the ankle joint to the neutral position per 2 seconds for 5 minutes a day, and between the mobilization, long leg cast was applied with the ankle in full plantar flexion state. The rabbits were all sacrificed at the end of the 6th week, and tension tests were performed using Instron 1000 testing machine. The following results were obtained: 1. Biomechanical properties such as break strength, stiffness, and energy absorption capacity were highest in the group which were mobilized during the 3rd week period, followed by the groups which were mobilized during the 2nd week, the 4th week and the 5th week periods in decreasing order. The biomechanical properties of the non-mobilized group and the 1st week-mobilized group were significantly lower than other groups. 2. There was no statistically significant difference of the biomechanical properties between the non-mobilized group and the 1st week-mobilized group. 3. The large gap formation was found in two out of eight tendons in the 1st week-mobilized group. There was no large gap formation in the other groups. Based upon the above results, following conclusions were obtained: 1. The intermittent passive mobilization during the fibrillogenesis period, which lasts from the postoperative 2nd week to 3rd week, was more effective in recovery of the biomechanical properties of the healing tendon than the mobilization during the other periods. 2. The intermittent passive mobilization during the inflammation period, which lasts from the immediate postoperative period to the postoperative 5-7th day, was not helpful in recovery of the biomechanical properties of the healing tendon, and it could bring about harmful effect such as large gap formation. In conclusion, the intermittent passive mobilization which started from the postoperative 2nd week, rather than from the immediate postoperative period, was more effective in recovery of the biomechanical properties of the healing tendon.