The effect of gravity and beam diameter on flame oscillation phenomena induced by external laser irradiation was investigated. Ethylene premixed gas was charged in a 50mm diameter combustion tube at 0.1MPa and a CO2 laser beam was irradiated along the center axis of the tube. The microgravity maintained during the experiment assures that presently observed oscillation phenomena are not caused by gravity. Experiments with different laser beam diameters indicate that the flame front curvature affects the oscillatory propagation behavior. The results suggest that this phenomenon is caused by a transformation of the flame front curvature by a locally increased flame speed observed with the laser absorption, which is closely related to the thermal-diffusive instability in combination with a flame stretch effect.