Experimental investigations were done in the paper for the process of venting explosion in a 0200 mm X 400 mm cylindrical vessel. Compared with the normal venting process, the phenomenon of external explosion was observed and discussed first. Moreover, when CH4-air mixture gases were used and the vent diameter was 55 mm, three kinds of condition were selected: phi = 0.8, phi = 1.0 and phi = 1.3. And two ignition positions were selected: at the vessel center and at the bottom. Then the venting processes influenced by these factors were experimented and discussed, too. (c) 2005 Elsevier Ltd. All rights reserved.