The effect of the environment (surrounding) conditions on the effectiveness of the heat exchanger for different working fluids are discussed here. The heat transfer between the heat exchanger external wall and environment are considered taken by radiation and convection. The convection heat transfer coefficient between the heat exchanger wall and the environment is taken as temperature dependence and the heat exchanger external wall is assumed a grey body. We analyzed the effect of the environment conditions on two type of heat exchanger working fluid, low viscid fluid (water) and medium viscid fluid (oil SAE-30). Because of the time is very effected factor on the heat transfer, then the unsteady state of the fluid inside the heat exchanger are assumed and analyzed in the present article. The increase of the emissivity, environment heat coefficient and environment temperature will decrease the hot fluid effectiveness and increase the entropy generation number. The fluid effectiveness is very effected by the environment specially when the working fluid has lower viscosity for same flow rate. The finite difference techniques is used to solve the unsteady state differential equations for the hot and cold fluid.