Unsteady behavior of buoyant diffusion flames which were observed at various gravity fields was studiednumerically by a finite volume method using the low-Mach number approximation and the temperaturedependent physical properties such as viscosity and diffusivity. The numerical model was a time-dependent and axisymmetric flow including a single-step chemical reaction. The objective was to clarify a periodic instability mechanism of buoyant diffusion flames such as flickering flame. The parameters of the jet injection velocity, V f , and dimensionless gravity level, G , were varied to clarify the effects of shear force and buoyant force on the unsteady motion of the flame. The gravitational effect was evaluated by varying the gravity acceleration ranging from 0.5G to 5G. An oscillatory frequency increased with an increase in gravity level. Convective flow in the flame caused by the vortical roll-up motion was enhanced in a highgravity condition. To elucidate the mechanism of the instability motion, axial velocity of convection, oxygen mass fraction, vorticity, and the distribution of radial density gradient were derived from the calculated results. It was found that the convective velocity and the oxygen mass fraction played a very important role in flame instability, And it seemed that the origin of unsteady motion was the generation of the vorticity. The maximum value of the vorticity estimated in high-gravity field was greater than that of the normalgravity field. Furthermore, the unsteady behavior caused by the buoyancy was correlated to the radial density gradient.