Heat transfer coefficient and fin efficiency of a heat exchanger dispersed in the microelement of control volume were predicted with various flow patterns, conditions and material properties. A computational program was developed by using the method of efficiency-NTU(Number of transfer unit). The modelling was applied to heat exchangers, also was integrated in power pack cooling system in an armored vehicle. The compatibility and the generality were proved by comparing the prediction values with the test results. The developed program may be useful for the design of the cooling system in an armored vehicle.