Abstract An experimental study was carried out to investigate the effect of microwave-assisted plasma ignition on laminar flame development in a 1.4 l constant volume combustion chamber (CVCC). The microwave-assisted plasma ignition system consisted of a commercially available 2.45 GHz magnetron (700 W), a waveguide, a 3-stub tuner, a mixer and a non-resistor spark plug. The combustion tests were performed using an acetylene–air mixture at a range of equivalence ratios and initial ambient pressures. The ejection timing of the microwave also varied based on the spark event. In-chamber pressure analysis and high speed imaging were combined in a CVCC to compare the results between the microwave-assisted ignition and the conventional spark ignition. The enhancement was evaluated from the combustion phase, combustion index, and the flame kernel size based on the in-chamber pressure results and shadowgraph images. Compared to the conventional spark ignition condition, the microwave-assisted plasma ignition showed an extended lean limit with the advanced combustion phase. The conventional spark ignition had a lean limit at the equivalence ratio of 0.6, while it was extended to 0.5 by the microwave-assisted plasma ignition under the initial ambient pressure of 0.1 MPa. The flame development time (time for 0–10% of total net heat release) for the microwave-assisted plasma ignition showed significant advancement especially in the lean mixture condition. The shadowgraph images indicated that the flame speed increased up to 20% with the microwave-assisted plasma ignition. In terms of equivalence ratios and initial ambient pressures, the enhancement was decreased with rich mixture and high initial ambient pressure conditions. In this situation, however, an early microwave ejection strategy was found to be beneficial to combustion showing a higher combustion index than the conventional spark ignition condition.