In this study, effects of microwave ejection on flame formation and development was investigated in a constant volume combustion chamber. The new ignition system was consisted with a commercial magnetron(2.45 GHz, 700 W) from a microwave oven, a mixing unit, a directional coupler, and a non-resistor spark plug. A series of experiments was performed under various equivalence ratio, ambient pressure and microwave ejection timing. The experimental results showed that microwave-assisted plasma ignition improved flame speed up to 20 % under the lean air-fuel condition. The lean limit could also be extended up to 0.5 with new system. However, enhancement was limited to lean and low initial ambient air condition. In terms of microwave ejection timing, an early microwave ejection strategy showed better combustion characteristics.