Quality of casting is dependent on the degree of casting defecets such as shrinkage, porosity, segregation, and etc. In this study, the sensitivity analysis was carried out to investigate the effect of casting parameters on the thermal history of the casting. Prediction of microshrinkage formation within the investment cast diesel valve seal ring was also made by computer simulation using ProCAST^(TM). The calculated thermal history was utilized to get the thermal gradient, the cooling rate, and the solidification velocity of the whole casting during solidification. In sensitivity analysis, the solidification time of the casting was largely dependent upon the metal pouring temperature and the thermal conductivity of the mould. The microshrinkage formation was closely related to the lower thermal gradient region in the Stellite alloy casting. The G/√R value, called Niyama criterion, successfully predicted the distribution of microshrinkage in the actual investment casting and the critical G/√R value for microshrinkage formation in Stellite alloy casting was found as 2℃ min.^(½)/㎝ for the diesel valve seal ring.