High mass flux subcooled flow boiling was investigated both numerically and experimentally in horizontal microtubes. Microtubes with inner diameters of similar to 600 and similar to 900 mu m, and outer diameters of similar to 900 and similar to 1100 mu m, and heated lengths of 6 and 12 cm were tested in order to investigate the effects of diameter and heated length on subcooled flow boiling at high mass and heat fluxes. In the experimental part, microtubes made of stainless steel were used, and deionized water was as the working fluid. In the numerical part, the two-phase Eulerian method was adopted using the finite volume approach. Numerical results showed a good agreement with experimental results. Heat transfer coefficients were higher in the microtubes with smaller diameters, while longer microtubes resulted in higher heat transfer coefficient. The results indicated that smaller pressure drops were achieved for shorter microchannels along with higher heat fluxes. Local heat transfer coefficients were presented along the microtube to provide an understanding on local flow boiling characteristics. As the vapor quality and void fraction increased, higher heat transfer coefficients were obtained. With the increase in mass flux, an enhancement in boiling heat transfer was observed implying convective heat transfer effects on flow boiling along with nucleate boiling. Furthermore, heat transfer coefficient increased with decreasing inlet subcooling. (C) 2015 Elsevier Ltd. All rights reserved.