The antioxidant effects of ascorbic acid (vitamin C), a-tocopherol (vitamin E), 2,6-di-t-butyl-4-cresol (BHT) and 4-hydroxy-2,2,6,6-tetramethyl-piperidine-N-oxyl (abbr. N-oxyl = 4-hydroxy-TEMPO) on the autoxidation of soybean oil (SBO) were evaluated at 140 ℃ in four different concentrations (at 1.0, 0.7, 0.5, and 0.3 wt% of SBO). Ascorbic acid and a-tocopherol showed lower active oxygen concentration at the range of 0.5.0.7 wt% concentrations than that at 1.0 wt%. However, on the contrary, 2,6-di-t-butyl-4-cresol and N-oxyl showed higher effect with increasing concentration. In 1.0 wt% antioxidant concentration, the inhibiting effect showed in the following order, 2,6-di-t-butyl-4-cresol > atocopherol > N-oxyl > ascorbic acid and in lower concentration of 0.5 wt%, the order was inverted as follows: ascorbic acid > a-tocopherol > 2,6-di-t-butyl-4-cresol > N-oxyl. The inversion of the order at lower concentration could be attributed to the synergetic and antagonistic effect arousing from hydrogen bonding. The composition of oxidized triglyceride was investigated by means of LC/ESI-MS/MS spectroscopy.