Evaporation heat transfer characteristics of $CO_2$/propane mixtures in horizontal smooth and micro-fin tubes have been investigated by experiment. The experiments were carried out for several test conditions of mass fluxes, heat fluxes, compositions of $CO_2$/propane refrigerant mixtures and tube geometries. Direct heating method was used for supplying heat to the refrigerant where the test tube was uniformly heated by electric current which was applied to the tube wall. Heat transfer coefficient data during evaporation process of $CO_2$/propane mixtures were measured for 5 m long smooth and micro-fin tubes with outer diameters of 5 mm, respectively. The tests were conducted at mass fluxes of 318 to 997 $kg/m^2s$, heat fluxes of 6 to 20 $kW/m^2$ and for several mixture compositions (100/0, 75/25, 50/50, 25/75, 100/0 by wt% of $CO_2$/propane). The differences of heat transfer characteristics between smooth and micro-fin tubes for various compositions of $CO_2$/propane refrigerant mixtures and the effect of mass flux, and heat flux on enhancement factor (EF) and penalty factor (PF) were presented.