The purpose of study is to identify the defect occurrence function of injection molded magnesium alloy products by using numerical simulation and to demonstrate its validity for practical application. The casting defects were classified into four types now line, crack, shrinkage and penetration, and its occurrence was investigated by using 38 types of actual casting products. The numerical simulation was also conducted to actual products previously mentioned to identify the defect occurrence function. The estimated defect occurrence function was applied to predict the casting defect of practical casting product. Test result showed that the good agreement between estimated defect occurrence function was also utilized to modify the casting mold for prevent the casting defects. Finally this paper proposed an effective concept to design the casting mold by using defect occurrence function.