Design of exhaust manifold is important due to its effects on performance of catalyst, thermal and thermo- mechanical loads. In this paper, first transient analysis of fluid was carried out with FLUENT software, then time-average temperature and heat transfer coefficient contour for three cycle were mapped on inner surface of exhaust manifold with MATLAB software for thermomechanical analysis. For validation of thermal analysis, experimental thermal data was used. In thermomechanical analysis, stress distribution in exhaust manifold due to thermal load, residual stress in exhaust manifold after cooling, vertical displacement of flange due to thermal load, flange torsion in vertical direction due to plastic strain has been validated. Finally, necessary geometrical corrections for optimal design structural resistance and strength of it has been delivered.