The coupling relationships between vented pressures and flames of micro- and nano-PMMA particles at elevated static activation overpressures were examined using the standard 20-L spherical dust explosion venting apparatus to reveal the differences of venting processes at different particle scales. It was found that by increasing the venting diameter or decreasing the static activation pressure, the maximum reduced overpressure of micro-PMMA particles was reduced significantly compared with that of nano-PMMA particles. By increasing the venting diameter, the competition between the pressure venting and flame venting was gradually transformed. For 100-nm PMMA particles, the vented flame would experience the advanced under-expanded jet flame, moderate under-expanded jet flame, subsonic jet flame and constant pressure combustion, with the latter two modes appearing only with a larger venting diameter. However, the venting process of 30-pm PMMA particles only underwent the moderate under-expanded jet flame, constant pressure combustion appearing only with a larger venting diameter. The flame venting accomplished later or earlier than the pressure venting depended on the venting diameter significantly. The vented flame scale and radiation intensity of the 100-nm PMMA particles were much stronger at the same concentration. (C) 2016 Elsevier B.V. All rights reserved.