This article presents an experimental investigation on condensation heat transfer of R-134a in horizontal straight and helically coiled tube-in-tube heat exchangers. The experiments were carried out at three saturation temperatures(35℃, 40℃ and 45℃) with the refrigerant mass flux varying from 100 kg/m2 s to 400 kg/m2 s and the vapor quality ranging from 0.1 to 0.8. The effects of vapor quality and mass flux of R-134a on the condensation heat transfer coefficient were investigated. The results indicate that the condensation heat transfer coefficients of the helical section are 4%-13.8% higher than that of the straight section. The experimental results were compared with the data available in literature for helical and straight pipes.