A theoretical analysis is developed to study the effect of radiative heat transfer on downward flame spread over a thin fuel in low gravity. The combustion model, which is an extension of that in Duh and Chen (1991). approximates gas—phase radiation using a two—flux method applied in the cross—stream direction. Numerical results show that there is a quenching limit in the low—gravity region which is inaccessible if radiation is neglected. The main controlling factor in low gravity (g < 0.05) is the conduction to radiation parameter (N∞) where the flame spread rate increases with an increase in Nee. Also, the Darnkohler number alone is found to be insufficient to characterize the flame spread behavior. On the other hand, in higher gravity the Damkohler number becomes dominant. Parametric studies are carried out by changing the gravity level and the ambient oxygen concentration subject to gas—phase radiation. Finally, a flammability map is constructed that combines the two effects.