This paper discusses the effect of diameter on both flow boiling heat transfer and transition from macro to microchannel evaporation. A recently proposed three-zone flow boiling model based on evaporation of elongated bubbles in microchannels is briefly described and used for the present analysis. In the microscale range, the model predicts an increase in the two-phase heat transfer coefficient with a decrease of diameter for low values of vapor quality and a decrease of the heat transfer coefficient for larger values of vapor quality. This behavior is explained by the influence of the liquid film thickness, deposited periodically behind passing liquid slugs.