Flame structure transitions of propane/air premixed combustion in a micro-planar quartz glass combustor in the present study have been extensively investigated using a high-speed digital camera. Six different flame propagation modes, namely, flame repetitive extinction and ignition (FREI), cellular flame, planar flame, U-shaped, inclined and spinning flames, are observed with varying inlet velocity and equivalence ratio. The FREI and cellular flames as well as spinning flames, for the first time, are experimentally discovered in micro-planar straight channel combustor. Based on the upper and lower limit of each flame mode, the flame structure regime diagram is constructed. Experimental observations indicate that different flame propagation modes may coexist during transition due to the hysteresis phenomenon. The effects of equivalence ratio, channel length and channel height on flame characteristics are analyzed. Results show that channel length has minimal effect on flame structure, whereas channel height significantly affects flame propagation behavior. It is also found that the flame dynamics is much more complicated with wider channel height. Furthermore, it is worthy of pointing out that the inclined flames disappears as the channel height is decreased from 3 to 2.5 and 2.0 mm, and the ultimate flame is U-shaped at large inlet velocity. (C) 2019 Elsevier Ltd. All rights reserved.