We recently confirmed the "tip open phenomenon" of lean H-2/air flame in a microchannel with cavities. The critical inlet velocity when fuel conversion ratio drops to 80% was defined as "flame splitting limit". In the present work, we numerically studied the impact of channel gap distance. Results showed that corresponding limits for 1.0-mm, 0.8-mm and 0.6-mm channels are 26 m/s, 33 m/s and 16 m/s respectively, exhibiting a non-monotonic dependence. The analysis reveals that when the gap distance is decreased from 1.0 mm to 0.8 mm, the proportion of fuel that involved into the cavities is increased, flame length is reduced simultaneously, and better preheating of the fresh mixture is attained. These positive effects lead to an increase in flame splitting limit. As the gap distance is further reduced to 0.6 mm, the excessive stretch effect results in complete extinction of downstream flame, causing a decrease of the splitting limit. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.