Anaerobic granular sludge plays an important role in high-rate anaerobic reactors for the improvement of wastewater treatment efficiency. The structure of granules is closely related to the mass transfer process and anaerobic biodegradation process. The current work aims at gaining an insight into the mechanism of crater formation during biogas production and then deepening the understanding of complex granule structure. The crater results mainly from the compression of biogas bubble and is relevant to the size of gas pore to some extent. The exerted compression pressure on a granule is estimated by the Young Laplace equation and compared favorably with the mechanical strength of granule measured by penetrometer. As porous materials, extremely small channels at nano- and micro-scale are well developed inside of sludge granule in favor of substrate transportation and biogas production. (C) 2016 Elsevier B.V. All rights reserved.