The preheating system is composed of the preheating tank, preheating steam lines, steam generators, and preheating down-comer lines. The main function of the preheating operation is to increase the temperature of the reactor coolant system(RCS) from room temperature to hot zero power condition using the heat from the reactor coolant pumps(RCPs) and the preheating tank heaters. The major parameters of the preheating tank design are the preheating tank volume, the preheating tank heater capacity, the velocity inside each pipe line, etc. In the present study, the numerical simulations of the preheating system were performed to check the availability of the preheating system design and the sensitivity analysis of each parameter was conducted.