The impact of the cavity depth on the combustion efficiency of lean hydrogen/air flames in a micro-combustor with dual cavities was numerically investigated. The results show that under lower and higher inlet velocities, the combustion efficiency varies in opposite tendencies with an increasing cavity depth. Namely, under lower inlet velocity, the combustion efficiency decreases as the cavity depth is increased, while it increases with an increasing cavity depth under higher inlet velocity. To reveal the underlying mechanisms responsible for these variation tendencies, we performed comprehensive analysis in terms of heat transfer from the flame to cavity walls, heat regeneration ratio via upstream wall, residence time in the cavity, and stretch rate at the flame tip. It can be summarized that the total heat flux at the cavity walls and the heat regeneration ratio via upstream wall are the two dominant factors under lower inlet velocity, while the stretch rate at the flame tip and the residence time in the cavity overwhelm the other two factors under higher inlet velocity. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.