In this paper, we will discuss whether modal analysis in hot condition is required to be analyzed or not. Exhaust system design has become a tough and challenging job to validate the structural integrity of a commercial vehicle. There is a need to reduce time in development cycle for a new product which has been a major challenge faced by auto makers to stay competitive in rapid growing automotive industry. Impacts of temperature effects on casted & tubular exhaust manifold are compared, which will be useful for valuable product design. For this, temperature distribution are mapped from CFD software and then analyzed in FEA software with structural boundary conditions in both hot and cold conditions. The exhaust manifold is close to the engine part in automotive exhaust system, because the cylinder discharge gas temperature can reach 800°C above, the tail gas heating effect is obvious. Because of the thermal stress, that caused by tail gas heating can be as high as hundreds of mega Pascal; it can also lead to thermal fatigue and cause structural fracture. Temperature has great influence on material mechanical properties, so it is necessary to take the influence of the temperature pre-stress on exhaust manifold/ hot end vibration characteristics into account.