Experiments of upward flow boiling heat transfer with water in a vertical smooth tube and a tube with axial microgrooves were conducted. Both of the tested tubes have a length of 2.5 m, an inner diameter of 15 mm, and an outlet diameter of 19 mm. The tube with axial microgrooves has many micro rectangle grooves in its inner wall along the axial direction. The grooves have a depth of 0.5 mm and a width of 0.3 mm. The tests were performed at an absolute pressure of 6 bar. The heat flux ranged from 0 to 550 kW/m(2) and the mass flux was selected at 410, 610, and 810 kg/m(2) s. By comparison, flow boiling heat transfer coefficients in the enhanced tube are 1.6 to 2.7-fold that in the smooth tube, while the frictional pressure drop in the enhanced tube is slightly greater than that in the smooth tube. The augmentation of flow boiling heat transfer in the tube with axial microgrooves is apparent. Based on the experimental data, a correlation of flow boiling heat transfer is proposed for the enhanced tube. Finally, the mechanisms of heat transfer enhancement are analyzed.