This study addresses the influences of low atmospheric pressure on heat and mass transfer process of downward flame spread over thick PMMA (polymethyl methacrylate) slabs in quiescent air. Series of experiments were conducted at three altitudes: Hefei (1.0 atm), Xining (0.77 atm) and Lhasa (0.67 atm). Burning rate (mass loss rate), flame spread rate and flame height were investigated in this paper. From experimental results in reduced pressure, burning rates are correlated by the expression: (m) over dot proportional to P-1.8. At lower pressure, it was found that flames go quenching below a critical Damkohler number, which is caused by chemical kinetic change and reduction of total heat feedback from flame to solid fuel. For finite width samples, the flame spread rate derived from experiments increased with the sample thickness, which is different from the previous conclusions based on an assumption of infinite width. Power-law progressions of flame spread rate and flame height to pressure were produced, and linear relationships between the index and thickness of samples were obtained. (C) 2013 Elsevier Ltd. All rights reserved.