An experimental investigation on two-phase flow boiling flow pattern, pressure drop and heat transfer of R-22, R-134a, R-410A and natural refrigerants of C3H8 and CO2 in horizontal circular small tubes is presented in this study. The experimental data were obtained over a heat flux range of 5 to 40 kW/m2, mass flux range of 50 to 600 kg/(m2·s), saturation temperature range of 0 to 15°C, and quality up to 1.0. The test section was made of stainless steel tubes with inner diameters of 0.5 mm, 1.5 mm and 3.0 mm, and lengths of 330, 1500, 2000 and 3000 mm. The effects of mass flux, heat flux, saturation temperature and inner tube diameter on the pressure drop and heat transfer coefficient are reported in this thesis. The experimental pressure drop and heat transfer coefficient were compared with the predictions given by some existing correlations. New correlations of pressure drop and heat transfer coefficient for small tubes that is based on the present experimental data are developed.