Heat transfer coefficient and pressure drop were measured in flow boiling of refrigerant R-123 in a uniformlyheated horizontal small tube of an inside diameter of 1.12 mm. Measured heat transfer coefficients increased with heat flux, while they showed very weak dependence on mass velocity and vapor quality. Such substantial dependence on heat flux and negligibly small dependence on mass velocity and vapor quality indicate that boiling heat transfer in small tubes is dominated by nucleate boiling. This is due to that the boiling number becomes rather high in small tubes when compared with large diameter tubes. Two-phase friction factor is found nearly constant irrespectively of the two-phase Reynolds number. Slug flow directly changed to annular flow of uniform thickness and stratified flow was not observed even at low mass velocity.