As a device to exchange heat from hot fluid to colder fluid, a heat exchanger is supposed to have high effectiveness. Theoretically, the increase of fluid rate would increase the effectiveness. Yet, it makes the contact time among the fluids become shorter. Based on this phenomenon, a research is carried on to find out how the fluid rate affects the effectiveness of a Shell-and-Tube Heat Exchanger. The working fluid used is air. It is found out that the effectiveness would up to a certain value if the fluid rate increases and then it would decrease. The effectiveness of a Shell-and-Tube Heat Exchanger would be higher if the hot air flows faster (in the tube side) and the cold air flows slower (in the shell side).