In this study failure analysis of a mini van diesel engine intake valve is analyzed in detail. Failure occurred at the groove of the valve. First mechanical and chemical properties of the valve are determined. After visual investigation of the fracture surface it is concluded that fracture occurred due to fatigue. Damaged valve, valve seat, cylinder block and piston surface is investigated and construed. Failure constituted 300 kilometers after the rectification of the engine. Therefore rectification and assembly steps of valve and engine parts are analyzed in detail. Considering assembly, design and working conditions of the engine, stresses occurring at the failure surface are calculated. Endurance limit and fatigue safety factor is determined. Because of high fatigue safety factor, reasons for failure are investigated and effects causing stress increase at groove of the valve are established. These effects are faulty assembly of the valve and valve seat causing a high impact effect. In conclusion effect of impact ratio on stress increase is explained.