Objective To investigate whether or not posterolateral rotatory instabil ity of the elbow is due to type-I and type-II coronoid process fracture together with anterior bundle of medial collateral l igament (AMCL) injury so as to provide a theoretic basis for its cl inical treatment. Methods Ten fresh-frozen upper extremities were collected from cadavera which was donated voluntarily with no evidence of fracture, dislocation, osteoarthritis, mechanical injury of the surrounding l igament and joint capsule. They included 9 males and 1 female with an average age of 25.1 years (range, 19-40 years), including 3 cases at left sides and 7 cases at right sides. All specimens were transected at the upper midhumeral and carpal levels preserving the distal radioulnar joints to get the bone-l igament specimens. An axial load of 100 N compressing the elbow joint was appl ied along the shaft of the forearm in the sagittal plane through the biomechanical study system. The load-displacement plot was measured and analyzed at elbow flexion of 90, 60, and 45° and under four conditions (intact elbow, type-I coronoid process fracture, type-I coronoid process fracture with AMCL deficient, and type-II coronoid process fractures with AMCL deficient). Results The posterior displacements were maximum at 90° elbow flexion. Hence, the results at 90° elbow flexion were analyzed: under condition of intact elbows, the posterior displacement was the smallest (2.17 ± 0.42) mm and the posterolateral rotatory stabil ity was the greatest; under condition of type-I coronoid process fracture, the posterior displacement was (2.20 ± 0.41) mm, showing no significant difference compared with that of the intact elbow (P gt; 0.05); under condition of type-I coronoid process fracture with AMCL deficient, the posterior displacement was (2.31 ± 0.34) mm, showing no significant difference compared with that of intact elbow (P gt; 0.05); and under condition of type-II coronoid process fracture with AMCL deficient, the posterior displacement was (2.65 ± 0.38) mm, showing a significant difference compared with that of intact elbow (P lt; 0.05). There was no macroscopic ulnohumeral dislocation or radial head dislocation during the experiment. Conclusion An simple type-I coronoid process fracture or with AMCL deficient would not cause posterolateral rotatory instabil ity of elbow and may not need to be repaired. But type-II coronoid process fractures with AMCL deficient can cause posterolateral rotatory instabil ity of elbow, so the coronoid process and the AMCL should be repaired or reconstructed to restore posterolateral rotatory stabil ity as well as valgus stabil ity.