In this study, we examined a method of predicting the flash formed in plastic injection molding via computer simulations. The gap between molds was calculated by mold deformation simulation, and the results were compared with the actual flash thickness. The molding test was performed using a mold manufactured from steel and aluminum alloy. The pressure and temperature on the cavity surface during molding were measured using a measurement sensor. An injection molding simulation based on the actual pressure and temperature was carried out, and the results were mapped to the elements of a finite element (FE) model for mold deformation analysis. In this model, a mold equipped with a molding machine was used for the mold deformation analysis. The results of the mold deformation analysis were in good agreement with the actual results. The flash thickness with the product molded with an aluminum alloy mold was smaller than that with a steel mold. The calculated gap amount and the position of its generation correlated with the actual formed flash. The possibility of the present method for predicting flash formation was demonstrated.