With the aid of same configured calorimeters as dimensions of 40% of that in ISO 9705 in Hefei (24 m, 100.8 kPa) and Lhasa (3650 m, 64 kPa), the influence of high altitude on heat release rate and combustion efficiency were investigated. Two groups of liquid pool fires of moderate sizes (D = 0.15, 0.25 m) with fuel level maintaining system were tested at two sites, respectively. Typical fuels with different sooting levels, i.e., N-heptane and Jet-A, were selected. The ambient air pressure effects were introduced by modifying the standard calculation method of heat release rate in ISO 9705. Experimental results indicated that the dimensionless burning intensity in the quasi-steady stage for both fuels could be accorded to pressure modeling with acceptable accuracy. And it could be correlated with radiation modeling well for Jet-A, while that of n-heptane failed and this may be explained by the flame convection feedback which could not be neglected for moderate sooty fuel of moderate sizes. The combustion efficiency at high altitude is slightly higher than that at atmospheric pressure, and it will gradually increase with the decreasing pool dimension regardless of the ambient pressure.