High tibial osteotomy is regarded as an effective treatment for unicompartmental arthrosis of the knee, but the optimal fixation method has not yet been devised. The purpose of this study was to confirm the superior strength of a rigid stepped plate as compared to a 4-hole L-plate device. Lateral closing-wedge high tibial osteotomy was performed on 30 ten-month-old porcine tibiae. Fifteen tibiae were fixed using a rigid stepped plate and 15 were fixed with a 4-hole L-plate. Three types of deforming force-compression, valgus bending, and varus bending-were each applied to 5 pair of tibiae fitted with either the rigid stepped plate or the L-plate using a mechanical testing apparatus. Values of load, displacement, and stiffness were analyzed to compare the fixation powers. Mean stiffness in the rigid stepped plate and L-plate groups were 977.8 (SD 230.3) and 836.2 (SD 248.4) N/mm for compression testing (P=.465), 38.7 (SD 12.9) and 21.6 (SD 9.6) N/mm for valgus bending (P=.047), and 62.3 (SD 18.1) and 29.8 (SD 9.7) N/mm for varus bending (P=.009), respectively. The rigid stepped plate has greater fixation strength than the conventional 4-hole L-plate. Given this improved level of fixation with the rigid stepped plate, immobilization duration after high tibial osteotomy could be shortened and complications reduced.