A theoretical analysis is developed to study the downward flame spread over the thermally-thin solid fuel vertically in the gravitational field. The combustion model is basically similar to that of Chen (1990). The effect of Damkohler number (Da), as a function of gravity level (g), is under investigation. Beyond the blowoff limit, the flame spread rate is found to be proportional to (g)-l/3. A combination of the isotherm and velocity vector distributions in gas phase and the solid fuel pyrolysis are presented together to illustrate flame structures at normal gravity. Subject to the same Da, the flame is weaker than that in forced convection. Within the elevated gravity domain of experiment, the computed flame spread rate agree very well with the measurements obtained by Altenkirch et al. (1980).