The antioxidative effect of doenjang (fermented soybean paste) and onion added in the seasoned pork, bulgogi was evaluated and the optimum mixture ratio of ingredients in seasoning sauce has been established using mixture experimental design (MED). When the seasoned pork, bulgogi was prepared with soy sauce 12% (control), doenjang 9% (in replacement of soy sauce 9%) added group (DG), and doenjang 9% and onion 33.4% (in replacement of water 33.4%) added group (DOG), and stored at $-25^{circ}C$ for 6 months, the peroxide value (POV) and thiobarbituric acid (TBA) value of control, DO, and DOG were all lower than that of control from the initial stage of storage. The antioxidative effect of seasoned pork was found to increase with the replacement of doenjang in seasoning sauce for pork bulgogi. Moreover, antioxidative effects in DOG increased more synergistically. Therefore, the lipid oxidation stability of pork bulgogi was improved by the addition of doenjang and onion onto the seasoning sauce. The optimum mixture ratio of seasoning sauce for pork bulgogi followed up by the MED was found to be doenjang 7.10%, soy sauce 9.46%, onion 19.72%, and water 42.58% with excellent sensory quality.