Vented 100 nm and 800 nm polymethyl methaaylate (PMMA) dust explosions were conducted in the standard 20 L spherical chamber with different venting diameters. During the pretty high pressure dust explosion airflow was vented out, the highly under-expanded jet flame with an appearance of "Mach disk" was formed. When the pressure and explosion airflow velocity gradually decreased, the highly under-expanding jet flame was degraded into the moderate under-expanding jet flame with the appearance of "Mach diamonds." With increasing venting diameter, the maximum reduced overpressure decreased, and the pressure venting rate increased. The maximum reduced overpressure of the 800 nm PMMA dust was initially higher with smaller venting diameters, and then the overpressure was lower than that of the 100 nm PMMA dust with larger venting diameters. These results indicate that there was a certain critical venting diameter beyond which the 800 nm PMMA dust explosions were more efficiently vented. Additionally, the larger pressure decline tendency of the 800 nm PMMA dust demonstrated that the 800 nm PMMA dust explosions were more sensitive to the venting diameter. The broken area of the venting membrane was roughly circular in shape with smaller venting diameters, whereas it was irregularly shaped with larger venting diameters. (C) 2015 Elsevier B.V. All rights reserved.