Abstarct: In this study flow boiling in a curved rectangular channel with the area of 16×60 〖mm〗^2 and the curvature ratio of 0.13227 has been investigated. Tests has been carried out for single-phase and two-phase flow. Single-phase tests were performed under conditions of: mass fluxes of 0.005 kg/s and 0.01 kg/s, heat fluxes of 5.56 kW/m2, 9.26 kW/m2 and 12.96 kW/m2, inlet temperature of 33.3°C. Two-phase flow tests were performed under conditions of: mass flux of 0.0075 kg/s and 0.01 kg/s, heat fluxes of 18.52-37.04 kW/m2, inlet temperature of 63.3°C. Distilled water has been used as working fluid. One side heating from bottom wall has been considered during test. The aim of this study is to obtain the heat transfer coefficient in both single-phase and two-phase flow in the curved channel. The results of single-phase tests showed that the Nusselt number is constant in the second half of the channel in all mass fluxes and heat fluxes which means thermal fully development in the flow. Also, Reynolds number in all single-phase tests is lower than 700, hence the flow regime is laminar. In flow boiling tests the maximum heat transfer coefficient of 0.01 kg/s mass flux is approximately 20.5% more than that of 0.0075 kg/s. The heat transfer coefficient for 0.0075 kg/s and 0.01 kg/s respectively ranges from 3000 to 4000 W/m2.°C and 2900 to 4500 W/m2.°C. Results of heat transfer coefficient variation by x vapor quality revealed that heat transfer coefficient might decrease with increasing heat flux. By visualization it is observed that the dominant flow patterns in two-phase flows are plug and slug and, in some cases, wavy flow has been observed too.