We reviewed the radiographs of 216 trochanteric fractures treated with a compression hip screw. In 2 cases, rotational displacement of the proximal fragment with the sliding of the screw was observed within 4 weeks after surgery. Both cases were categorized into type 1, grade 4 fractures according to Evans' classification. The reduction was inadequate, and the axis of the screw was not parallel to that of the femoral neck in the lateral view of the rediograph. Using a fracture model we demonstrated the mechanism of the rotational displacement of the proximal fragment simultaneously with the sliding of the screw. When using the compression hip screw, reduction of the flexion or extension displacement and placement of the screw parallel to the femoral neck is necessary to avoid such displacements after surgery.