Abstract: The experimental study on flow boiling patterns and heat transfer of R134a in horizontal helically-coiled pipe was presented. The experiment was carried out at saturation temperature from 5 to 15 ℃, with the refrigerant mass flux varying from 50 to 500 kg·m^-2·s^-1, heat flux varying from 5 to 20 kW·m^-2, and the vapor quality ranging from 0.01 to 0.9. The flow patterns were observed and analyzed through visualization experiment. It is found that the flow patterns in the upflow and downflow sections are different under the same conditions, especially the transition flow patterns before the annular flow are obviously different, which are wave annular flow and super slug flow in the upflow and downflow sections, respectively, so two different flow pattern maps are proposed for the upflow and downflow sections, respectively. The effects of vapor quality, mass flow rate and heat flux on the boiling heat transfer coefficients were obtained, and the new correlation was developed for predicting the flow boiling heat transfer coefficients of R134a in horizontal helically-coiled pipe.