The flow boiling heat transfer and frictional pressure drop of R32 were experimentally investigated inside three horizontal small-diameter microfin tubes of equivalent diameters of 2.1 mm, 2.6 mm, and 3.1 mm. These microfin tubes of the same fin height, helix angle, and number of fins, but different tube diameters were used to resolve the effect of tube diameter. Furthermore, the effects of the mass velocity, heat flux, vapor quality, and channel size on the flow boiling characteristics were investigated. It was found that under high-mass velocities and high-heat flux conditions, the well-known heat transfer characteristics, forced convection and nucleate boiling, were dominant. On the contrary, under intermediate mass velocities and low-heat fluxes, the heat transfer through a thin liquid film formed in spirally grooves was dominant. The heat transfer coefficients increased and the dryout inception quality decreased with a decrease in channel size, under almost all conditions. Moreover, the frictional pressure drop increased with an increase in mass velocity and vapor quality and a decrease in channel size. (C) 2019 Elsevier Ltd. All rights reserved.