A numerical model is developed to simulate flame spread over a thin solid fuel horizontally in a gravitational field. In this paper, the burning behaviors in normal and elevated gravity are analyzed. In normal gravity, the flame structures are investigated in detail in order to find the essential features of such spreading flame. The flame can be separated into an upper flame and a lower flame by the fuel plate. There are three high temperature regions: two of them are at the upper and lower flame fronts, respectively, and the other one is at the combustion plume, just behind the burnout point. The induced flow moves upward and forms a thermal plume above the fuel plate. An entrainment of air moves toward the flame, opposed to the upper spreading flame. Below the fuel plate, the induced flow moves concurrently with the lower spreading flame. From the reaction rate distribution, the characteristics of triple-reaction-zone is identified. The heat flux on the bottom side is more intensive and extends further upstream than that on top side, confirming that the controlling mechanism is determined by the spreading flame underneath the fuel plate. A qualitative comparison with experiments is given. A parametric study by elevating gravity is followed. A blow off limit occurs at about g = 5.5. During the process, the horizontal flame spread rate is found to be slower than the downward one behind g = 2.5, where the lower flame is broken due to the up-rising induced flow. The variation in flame structures as a function of gravity is discussed.