Warpage issue of injection molding product is widely concerned by designer and molder because it is critical to product quality. How to control warpage within tolerance is the ultimate goal for injection molding industry. Accurate computer aided engineering (CAE) warpage prediction helps designer to determine the best design from different prototypes quickly at the beginning of development or helps molder pinpoint the cause of trouble during manufacturing. However, the complexity of warpage arises from several factors during injection molding like material, injection machine, part and mold design. An accurate warpage prediction must take these factors into consideration comprehensively. The real machine response is compared with filling pressure to verify flow analysis result is accurate enough as input parameter of following warpage prediction. Unlike linear warpage calculation simply based on material PVT property, Moldex3D solver considers material viscoelasticity to simulate the significant modulus change when polymer transits from rubbery phase to glassy phase. Together with in-mold constraint and free deformation after ejection in warpage calculation, the warpage prediction shows high agreement with real box product on three different materials, PS, PC and PP. Keywords: Viscoelasticity, Warpage, Machine response