A numerical analysis using an unsteady combustion model is presented to study the ignition and subsequent downward flame spread over a thermally thin solid fuel in a gravitational field. The solid-fuel temperature rises gradually in the heat-up stage and the pyrolysis becomes more intense. Ignition, including the induction period and thermal runaway, occurs as soon as a flammable mixture is formed and the gas-phase temperature, heated by the solid fuel, becomes high enough. During the induction period, the reactivity and temperature in the gas phase are mutually supportive. The thermal runaway consists of a burning premixed flame as the flow moves with the flame front. This is followed by a transition from a premixed flame into a diffusion flame. The flame front extends along and toward the upstream virgin fuel as the diffusion flame is formed. Finally, steady flame spread takes place as burnout appears. The ignition delay time is found to be controlled mainly by the time required to form the flammable mixture and is almost independent of the gravity level and the ambient oxygen index. The ignition delay time increases nearly linearly with an increase in solid-fuel thickness within the range of 0.005 cm less than or equal to (τ) over bar less than or equal to 0.02 cm and is proportional to ((Q) over bar (max))(-1.11) within 2W/cm(2) less than or equal to (Q) over bar (max) less than or equal to 8 W/cm(2). The steady downward flame-spread rate decreases with increases in the gravity level or fuel thickness and with decreases in the ambient oxygen index but is independent of the incident peak heat flux. The blowoff limit is around 6.7 (g) over bar (e) and the extinction limit is found to be Y (Oinfinity) = 0.131.