Objective To compare biomechanical effects of augmentative plating assisted by different types and numbers of intramedullary nails in the treatment of nonunion of femoral shaft fracture after intramedullary nailing.Methods Twenty-five specimens of synthetic femur were randomly divided into an experimental group (20 bones) and a control group (5 bones).All the bones were made into models of nonunion of femoral shaft fracture after intramedullary nailing.The control group was subjected to no augmentative plating.The experimental group was further divided into 4 subgroups (n =5) which were subjected to fixation respectively by:a 7-hole plate with 3 bicortical screws on each side,a 7-hole plate with 3 unicortical screws on each side,a 5-hole plate with 2 bicortical screws on each side,and a 5-hole plate with 2 unicortical screws on each side.The torque values of all the subgroups were measured and compared when the models were subjected to torsional angles of 5°,10°,15°,20° and 25°.Results The anti-torsional strengths at torsional angles of 5°,10°,15°,20° and 25° in the 4 experimental groups were all significantly greater than in the control group(P ＜ 0.05).At a torsion angle of 25°,the mean torque value in the 3-bicortical-screw group (46.51 ±6.66 nm) was significantly greater than in the 2-bicortical-screw group (39.03 ±8.52 nm) (P ＜0.05).At torsion angles of 5°,10°and 25°,the mean torque values in the 3-unicortical-screw group (13.48±1.68 nm,23.29±3.72 nm and 48.19±5.34 nm,respectively) were significantly greater than in the 3-unicortical-screw group (9.56 ± 3.62 nm,16.99 ± 5.49 nm and 38.69 ± 6.56 nm,respectively) (P ＜ 0.05).At torsion angles of 20° and 25°,the mean torque values in the 3-unicortical-screw group (39.15 ± 5.07 nm and 48.19 ± 5.34 nm) were significantly greater than in the 2-bicortical-screwgroup (32.50 ± 5.65 nm and 39.03 ± 8.52 nm) (P ＜0.05).Conclnsions The anti-torsional strength of fixation by 3 screws on each side is significantly greater than by 2 screws on each side.When the number of screws is the same,there is no significant difference between fixation by unicortical locking screws and that by bicortical conventional screws.Augmentative plating with 3 unicortical screws or 3 bicortical conventional screws on each side is recommended for treatment of nonunion of femoral shaft fracture after intramedullary nailing.