An artificial knee joint includes a lower joint member, an axle, a kneecap member, and a press member. The lower joint member is formed with a horizontally extending axle-receiving hole, and a clearance extending from and in spatial communication with the axle-receiving hole. The axle is received in the axle-receiving hole. The kneecap member is pivoted to the lower joint member, and is adapted to be connected to a prosthetic thigh. The press member is mounted to the kneecap member for pressing the lower joint member when the kneecap member pivots in a direction relative to the lower joint member. Therefore, the clearance and the axle-receiving hole are constricted to result in tightened connection between the lower joint member and the axle and resistance to relative rotation between the lower joint member and the axle.