This study investigates a downward propagating cellular flame to describe its transient phenomena induced by CO2 laser irradiation. An experimental investigation has been conducted in a combustion tube with C2H4 - O-2 - CO2 fuel rich mixtures at ambient temperature and atmospheric pressure. After ignition, a cellular flame is formed, and once CO2 laser irradiates the mixture, several motions can be observed. First, transition from cellular to smooth surface occurs as a consequence of the local increment of the flame speed. Then, as the laser is continuously irradiating, concave structure develops and finally new cells appear around this concave structure because buoyancy overcomes the initial enhancement of the propagation speed, decelerating it close to the initial value where cellular flame can exist. Therefore, it is concluded that CO2 laser actively controls the evolution of the propagating flame shape.