Experimental study on two-phase flow boiling heat transfer of R-410A and R-134a in horizontal small tubes was reported. The pressure drop and local heat transfer coefficients were obtained over heat flux range of 5 to 40 kW m -2, mass flux range of 70 to 600 kg m-2 s-1, saturation temperature range of 2 to 12°C, and quality up to 1.0. The test section was made of stainless steel tube with inner tube diameters of 0.5 and 3.0 mm, and lengths of 0.33, 2.0 and 3.0 m. The section was heated uniformly by applying an electric current to the tube directly. The effects on pressure drop and heat transfer coefficient of mass flux, heat flux, tube diameter, and saturation temperature were shown. The experimental results were compared against several existing correlations. A new boiling heat transfer coefficient correlation based on the superposition model for refrigerants in small tubes was developed.