In this paper, a coaxial cylindrical Dielectric Barrier Discharge (DBD) reactor is designed to investigate the enhancement processes of plasma-assisted methane combustion with activating premixed CH4/air mixtures. Flame shape and flame front structure are observed by digital camera and OH Planer Induced Fluorescence (OH-PLIF) technique, respectively. With plasma being off and on, the temperature profiles along the axis of flame are measured by a double platinum–rhodium thermocouple and compared with each other. Experimental results show that, without plasma assistance, the flame appears as conic shape. While plasma is applied, the height of conic tip decreases dramatically. Due to the height of flame edge not decreasing obviously, the flame exhibits an ‘M' shape structure. The highest temperature position moves towards the tube when plasma is applied. But temperature value does not increase obviously.