The main objective of this work is to study the mean flame height and flame radiative heat flux of medium scale thermal buoyancy source with different aspect ratios in a sub-atmospheric pressure at high altitude, which has not been investigated before. Two series of radiative heat flux experiments of medium scale thermal buoyancy source were conducted separately in Hefei (altitude: 50 m, pressure: 100 kPa) and Lhasa (altitude: 3650 m, pressure: 64 kPa). The mean flame heights of rectangular heat sources burners were obtained by image processing method and the radiation flux was measured by a water cooled wide angle radiometer. Four medium scale rectangular thermal source burners with same surface area (about 420 cm(2)) but different buoyancy source dimension aspect ratios n (n = L/W, long side divided by short side) were used to produced heat sources. It is found that the flame height in reduced pressure is higher than that in normal pressure, and the flame radiation flux increases with increase in source aspect ratios. Meanwhile, the flame radiation fraction of medium scale thermal buoyancy source with different aspect ratios in a reduced atmospheric pressure changes little with ambient pressure and theoretical demonstrate with expression chi(R) similar to p(0.45)sigma T-F(4). (C) 2015 Elsevier Ltd. All rights reserved.