PURPOSE: To evaluate the relationship between the length changes of both forearm bones and function of wrist. To know permitted length discrepancy for good wrist function after operation in fracture of both bones of forearm MATERIALS AND METHODS: From Jan. 1995 to Dec. 2000, 21 cases were followed over 1 year, were treated with compression plate and screws due to fracture of both bones of forearm in our hospital. Mean duration of follow-up was 3 years 6 months. The postoperative length difference was compared to preoperative or unaffected side in roentgenography. Four groups were defined to A, B, C and D by postoperative length difference ; ≤1mm, 1 2mm, 2 3mm, and >3mm for comparison. The function of wrist joint was evaluated with the Anderson 's classification and Mayo modified wrist score. RESULT: Group A were 11 cases(52.3%), B 5 cases(23.8%), C 4 cases(19.0%) and D 1 case(4.8%). By the Anderson 's classification, the number of Excellent were 11 cases(52.3%), Good 7(33.3%), Fair 3(14.3%). In the group of the length difference lesser than 2mm, the number of Excellent were 11, and Good 5. The Mayo modified wrist score was 75.15 in the group of the length difference lesser than 2mm, that was higher than 61.15 in the group of more than 2mm. CONCLUSION: To obtain a good wrist function after operative treatment of fracture of both bones of forearm the length discrepancy of both bones should be lesser than 2mm.