The propagation of methane/air and hydrogen/air explosion flames in four closed ducts with different aspect ratios was numerically investigated using large eddy simulation (LES). The numerical model was validated by comparing the numerical results with the experimental results, and reasonable agreement is observed between them. For methane/air, only the tulip shaped flame (concave flame front) can be observed after the flame inverts and no oscillation occurs. The tulip lips distortion occurs in the duct with aspect ratio (the ratio of length to width) of 7.5 in hydrogen/air, and the deformation becomes more distinguished as the aspect ratio increases. Consistent with the flame structure evolution, the flame speed and rate of pressure rise decrease after the flame touches the sidewalls in methane/air. For hydrogen/air, with the emergence of tulip lips distortion, an oscillation appears in the flame propagation speed and pressure curves when the aspect ratio is 7.5. Moreover, the oscillation becomes stronger as the aspect ratio increases. With this increase in the aspect ratio, the interaction between flame and pressure wave leads to the emergence of different flow patterns after the tulip shaped flame formation. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.