Injection molding tools are expensive and the fatigue failure during production would result in very costly rework on the tool and downtime. Currently, mold designs are mostly based on expert experience without a careful stress analysis and the mold tool life cycle relies largely on rough estimates. The industry state of the art applies averaged temperature change and peak pressure load on the mold tool. The static analysis is then performed. Mold temperature history and thermal shock are not considered in the durability analysis. In this paper, a transient thermal analysis of the tool is performed in conjunction with the injection molding process simulation. The spatial and temporal variation of temperature, pressure and clamping forces are exported from Moldflow simulation. These histories of temperature and pressure are converted to appropriate loading curves and mapped into Abaqus FEA model. A transient FEA simulation is performed in Abaqus for the length of the cycle time with temperature and pressure variation over the entire tool incorporated as loads. Thermo-mechanical stresses are predicted along with contact interactions. These stress histories are used for more accurate fatigue life prediction of the mold tool. This new methodology would help to avoid the risk of fatigue failure for injection molding tools. Costly tool rework and downtime would be avoided.