The combustion characteristics of non-premixed hydrogen micro-jet flames in co-flow air were investigated in a wide fuel flow velocity range experimentally and numerically to extend the knowledge on non-premixed micro flame. The direct flame images were captured by a digital reflex camera with long exposure time, and the OH radical distributions in the flames were measured by the laser induced fluorescence technique. The experimental observation shows that the flame height and shape change with the deceasing fuel flow velocity. A detailed chemical reaction mechanism was employed in the numerical computations with considering the heat transfer between gas and solid micro tube. The computed distributions of OH concentrations agree qualitatively with those obtained by the experiments. The flame structure and thermal interaction between the flame and solid tube were inspected in detail. It was found that the thermal interactions at high, intermediate and low fuel flow velocities are different. The details of the thermal interactions between flames and solid tube are depended on the shape of the micro flame, which essentially affects the temperature distribution. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.