In order to obtain a higher combustion efficiency and a lager heat transfer from outer wall, a new type of micro cylindrical combustor with suction pipe, mixing pipe, diffuser pipe and shrinkage pipe and the combination of cavity and backward-facing step is designed and a numerical investigation on non-premixed hydrogen/air reacting flow inside three micro combustors has been carried out at atmospheric pressure. Moreover, the combustion characteristics and working performance of micro combustors are also investigated. Results show that the cavity and the backward-facing step in micro combustor is useful for the heat recirculation and the flame stability. Moreover, the combustion efficiency and the heat transfer from outer wall can be improved by increasing the length of combustion chamber within limited range. A high and uniform outer wall temperature distribution is desirable for the L-2 micro combustor, therefore, the L-2 micro combustor is more suitable for the micro thermo photovoltaic (MTPV) system when hydrogen volume flow rate is equal to 150 mi./min and equivalence ratio phi is equal to 1.0. C) 2016 Elsevier Ltd. All rights reserved.