An experimental study was performed in a rectangular duct to investigate the detonation diffraction process in stoichiometric methane-oxygen mixtures. A wedge was placed in the duct to form two channels between the wedge and the channel walls, i.e., an upper channel and a divergence channel. The upper channel heights were 27 mm, 47 mm and 67 mm, and the deviation angles (alpha) were 60 degrees and 30 degrees. On the bottom wall and side wall, soot foils were placed to record the cellular structure evolution. For the larger deviation angle (alpha =60 degrees), the re-initiation mechanisms can be varied, which are affected by the upper channel height. Two over driven detonation regions are generated behind the wedge on the bottom wall as well as the side wall for more sensitive mixtures (characterized by smaller detonation cell sizes). However, for insensitive mixtures, the re-initiation process seems to be more complicated. More regions containing finer cells can be observed on the foils. In case of alpha =30 degrees, the detonation re-initiates at the divergence wall and propagates toward the side wall. Universally, the re-initiation is facilitated as the upper channel height increases, and equivalently, the re-initiation distance is reduced. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.