Nowadays, computer simulations are becoming more and more important in performance investigation of thermal systems. In this article, radiator of cooling system of diesel engine of ER24PC locomotive is simulated. The radiator is composed of parallel and series arrangement of compact heat exchangers with offset strip fins. It also has two high and low temperature sections. Due to the complexity and compactness of heat transfer plates implemented in the radiator, the simulation is carried out in two steps. First, a relation for coolant-side and air-side heat transfer coefficient is correlated using computational fluid dynamics. Due to vortex shedding phenomenon in the staggered fin arrays, governing equations are solved transiently in two-dimensional space. Appropriate timestep for the transient solution is chosen according to time period of vortex shedding from the surface. In the second step, using the developed computational code, the overall thermal performance of the radiator is simulated as a heat exchanger. Consequently, temperature distribution inside the radiator and its thermal performance are studied. Amount of heat released from the radiator in different flow rates and temperatures of fluid flowing out of radiator are among the outputs of the developed code. Finally, thermal performance curve of radiator is obtained.