The unsteady behavior of buoyant methane diffusion flame is simulated numerically. The turbulent transport and the reactive mixture are evaluated using a RNG - k - e - g turbulence modeling and a presumed shape (?-function) pdf approach. A fine description of radiation is achieved by using a two equation sub models for the description of soot formation and a differential PI-approximation to calculate the irradiance field. The numerical results show that the buoyant flow above the flame is characterized by the development of large eddies on both sides of the column formed during the expansion of the hot gases. The growth of these buoyancy driven instabilities is also marked by an oscillatory behavior as it can be noticed on the time evolution of the flame height.