This research examines effects of molding conditions on residual stress and tensile strength of injection molded plastic parts. Six factors (injection pressure, melt temperature, mold temperature, packing pressure, packing time and cooling time) and three levels of each factor were examined. Taguchi experimental setup was used to reduce the number of experiments. The rectangular shaped flat parts were molded using high density polyethylene (HDPE) and polypropylene (PP) samples. The layer removal technique was used to prediction of the residual stresses. Results show that the influential factors are cooling time, melt temperature and packing pressure on HDPE parts, and cooling time, melt temperature and mold temperature on PP parts.