Using a narrow width rectangular open top tray, we performed flame spread experiments over several different liquids under various oxygen concentrations. The initial liquid temperature was kept below the flash point for all experimental cases, therefore, sub-surface liquid circulation was formed during the flame spread. We measured the flow structure by a particle-track laser-sheet (PTLS) combined with a high-speed video camera and the temperature structure by a diffusion light shadowgraph (DLSG) technique. The flame spread rate in normal gravity with X O 2 = 0.175 matched that in microgravity (10–5 g) with X O 2 = 0.21. The size of thermal and momentum boundary layers created by a sub-surface liquid convection increased with a decrease in both oxygen concentration and gravity. The depth of the momentum boundary layer was found twice larger than the depth of the thermal boundary layer. Scaling analysis was conducted and found that the non-dimensional flame spread rate can be correlated with ( Ma· Pr) −1 for the shallow liquid pool and with ( Ma· Pr) −1.5 for the deep liquid pool. Here Ma is Marangoni number and Pr is Prandtl number. We predicted that ( Ma· Pr) −1 decreases with a decrease in gravity and experimentally found that ( Ma· Pr) −1 decreases with a decrease in the ambient oxygen concentration. A similarity was found between the dependency of ( Ma· Pr) −1 with gravity and that with the ambient oxygen concentration, indicating that the microgravity flame spread over liquid can be studied under normal gravity with a reduced oxygen concentration ambient.