The buoyancy-free environment in microgravity alters spacecraft fire behaviour compared to Earth’s normal gravity. The diffusion flame spreads by preheating the fuel in front of the flame, where the neat heat transfer rate to the preheat region in the solid fuel determines the flame spread rate. This heat transfer comprises conduction, convection, and radiation. However, investigating these heat transfer modes that influence flame spread behaviour in 1 and 0g is crucial. A steady 3D numerical model simulates flame spread phenomena over a thin cellulose fuel of 2 cm width and 8 cm width at low opposed flow velocity (here 5 cm/s) for oxygen concentration ranging from extinction to 40% in 1 and 0g. Computations show that conduction dominates in both gravities. Nevertheless, in 2 cm width fuel, radiation contribution to the net heating rate increases in microgravity with the increase in oxygen molar concentration and decreases in normal gravity. However, radiation contribution increases with oxygen molar concentration in 8 cm width fuel in microgravity and normal gravity. Radiation is also responsible for flame extinction at low flow speeds.