A comparative study was performed to investigate flame propagation in a square-cross section channel filled with either flexible- or rigid-obstacles with blockage ratio (BR) of 0.429. Experiments were conducted in premixed hydrogen-air mixtures with different equivalence ratios, at initial conditions of 100 kPa and 298 K. High-speed Schlieren photography was used to obtain the detailed flow structure, flame front evolution and the flame tip velocity. Also, pressure transducers were employed to monitor the pressure around the obstacles. Flame propagation across the obstacles was found to be strongly affected by flow contraction induced by obstacles and separated flow pattern downstream of obstacles. Flame propagation with rigid obstacles is mainly governed by the turbulent burning of the fresh gas in the pockets. For the flexible cases, the flow structure is characterized by the shear layer coming off the obstacles leading-corner and the vortex downstream from the obstacles. These special flow structures together provide a flow contraction and constrict flame propagation in the obstacles-free channel, and therefore the flame maintains acceleration. Most notable, the gas flow ahead of the flame purges the flexible obstacle to tilt, yielding an increase in BR, which is correlated with the stronger acceleration as the flame propagates through the obstacles. However, exposing the obstacles to the overpressure for a long period also induces too much deformation. Therefore, the instantaneous BR (BRreal) will also decelerate slightly. Interestingly, BRreal is closely related with the overpressure level. (C) 2018 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.