Combustion behavior of diffusion flames in laminar regime was investigated. Experiments of gas jet diffusion flames in quiescent air were made under normal and microgravity conditions for various burner diameters and for various fuel flow rates. In normal gravity, periodic oscillating flames were observed. On the other hand, the flames were stable in microgravity. Frequency analyses under both conditions reveal that flames oscillate within 10 to 20 Hz in normal gravity and flame oscillation is suppressed under microgravity. The frequency of the flame flickering is sensitive to the fuel volume flow rate for small burner diameter. It was confirmed that Strouhal number and Richardson number could be used as criteria for oscillation behavior. It implies that the frequency might be proportion to the gravity to the power of 0.7.