A detailed three-dimensional numerical model is used, to study the effect of fuel Lewis number (LeF = α/DF) and mass transfer on flame spread over thin solids. The focus of this work is to investigate the sensitivity of opposed flame spread rate to fuel Lewis number in microgravity. By using computational model as a tool, the effects of fuel mass diffusion in three directions are isolated by means of fuel Lewis number, LeF. In general, LeFx = LeFy = LeFz = LeF however in this study Lewis numbers in x, y and z directions are varied independently. Sensitivity analysis is performed to quantify the influence of LeFx, LeFy and LeFz on flame spread rate. Numerical simulations are performed to analyze the behavior of spread rate with LeF for 2D limit and 2cm wide fuel. © 2018 Combustion Institute. All Rights Reserved.