The article aims at explaining the effects of L/D (the ratio of length to diameter) on premixed methane/air flame propagation in the closed duct, which is based essentially on the experimental and numerical methods. High-speed camera, pressure transducer, and large eddy simulation model are used to study the flame shape changes and pressure build-up in the closed ducts with various L/D. The results demonstrate that the premixed flame propagation undergoes four typical stages, namely spherical flame, finger-shaped flame, flat flame, and tulip flame. The pressure growth rate and the flame tip speed reach the maximum value simultaneously when the flame lateral sides touch the sidewalls in the closed duct. The dynamic synchronization of the flame tip speed and the pressure growth rate indicates the tulip flame is a purely hydrodynamic phenomenon resulting from the interaction of the flame front and the pressure wave. Particularly, the maximum flame tip speed increases linearly with increasing L/D. (C) 2015 American Institute of Chemical Engineers