Locking of the metacarpophalangeal(MP) joint of thumb in mild hyperextension is relatively uncommon problem. The most frequently recognized cause appears that proximal palmar ligament or volar plate were ruptured tranversely with its distal part riding over the volar prominence of the radial condyle of the first metacarpal head. We experienced two cases of locking of the MP joint of the thumb due to volar plate injury. Closed reduction was failed in both cases and open reduction was done by cutting the constricted ligament hundle over the radial condyle of the first metacarpal head. Our description of this mechanism is supported by expeimental evidence that we obtained from the cadeveric specimens. The height of radial side condyle of first metacarpal head is higher than ulna side of that by 1.76mm in both hands of 16 cadeveric specimen. So we are going to report these cases with a review of the literature, and suggest that the preferable location of surgical incision in open reduction is radial side of metacarpal head.