A numerical investigation is carried out to understand the role of fuel Lewis number (LeF=α/DF) on flame spread over thin solid fuel in a quiescent microgravity environment. A three-dimensional numerical model was used wherein fuel Lewis number, LeF was split into directional components along and perpendicular to the fuel surface to elucidate the directional effect of fuel vapor diffusion on flame, spread rate, and its extinction. It is shown that fuel vapor diffusion in the direction perpendicular to the fuel surface plays the most dominant role in the flame spread and extinction behavior. With this approach, the non-monotonic variation of flame spread rate with LeF, influence on limiting oxygen index are also explained. The spreading flame exhibited diffusive-thermal (D-T) instabilities near-extinction. The flame extinguished forming cellular type flame for LeF <1 and developed oscillatory behavior for LeF >1. It is shown that D-T instability induced in one direction triggers the instability in other direction as well. Sensitivity analysis revealed that fuel diffusion becomes increasingly important for narrow fuel strips. © 2020 The Combustion Institute