Natural gas has been considered as one of the most promising alternative fuel for Transportation due to its ability to reduce regulated pollutants, cost and as safe as other fuels like gasoline and diesel. This thesis expresses about cooling system of converted CNG engine. In converted CNG engine, the engine cooling system has been increased because a medium working temperature has been increased. Engine cooling system is closely related with overall performances of an engine, including reduction of fuel consumption, decrease of air pollution, and increase of engine life. Radiator is the heart of cooling system. Compact heat exchangers have been widely used in various applications in thermal fluid systems including automotive thermal management systems. Radiators for engine cooling systems, evaporators and condensers for heating ventilation and air conditioning (HVAC) systems, oil coolers, and intercoolers are typical examples of the compact heat exchangers that can be found in ground vehicle. Among the different types of heat exchangers for engine cooling applications, cross flow compact heat exchangers with flat tube and flat plate fins are of special interest because of their higher rejection rate capability with the lower flow resistance.