<p>The mechanism and steady propagation of upward flame is key aspects of the solid materials flammability. The controlling mechanisms of upward flame spread over solid fuels were studied through a numerical modeling. The objective of the present study is to gain insight into the intrinsic mechanisms of the steady spread process of a solid fuel, as well as to gain a more comprehensive understanding of the critical conditions for the steady spread process. For this purpose, a two-dimensional numerical model has been developed using the Fire Dynamic Simulator (FDS6) code, in which both solid-phase and gas-phase reactions are calculated. Flame spread process with a series of gravity levels were studied, and the buoyant flow effects were discussed. The simulation results show that with the decrease of the buoyant flow, three distinct flame spread modes are identified, namely the skipping mode and splitting mode due to advanced pyrolysis of downstream fuel, and steady spread mode for subcritical gravity. When the buoyancy is weak enough, the flame can spread upward with a steady rate, which is dominated by the constant air supply caused by the backflow zone aside the flame. Under these conditions, the flame spread rate increases linearly with the increased gravity. Reduced-pressure experiments were carried out to examine the flame spread modes under suppressed buoyancy conditions, which shows good agreements with numerical results.</p>