This paper reports an interesting experimental phenomenon regarding flame radiant fraction and flame volume of laminar diffusion methane flames under sub-atmospheric conditions. The quantitative formula of flame volume was derived by assuming that the shape of the flame was cylindrical, with which the relation between flame volume and ambient pressure was obtained. Then the dependence of flame radiant fraction on ambient pressure was analyzed, and the flame radiant fraction was seen to have a quarter-power law dependence on ambient pressure. Experiments were conducted using a circular nozzle (diameter of 0.012 m) at 45-100 kPa in a confined cabin with internal dimensions of 3 m x 2m x 2m. Fuel mass flow rate was set to 5.98-20.91 mg/s with jet exit Reynolds numbers of 57.66-201.8 and Froude numbers of 0.046-2.796. Experimental results obtained show an inverse relationship between flame volume and ambient pressure; the flame radiant fraction was at 0.194 power of ambient pressure; and the total absorption coefficient was at 1.78 power of ambient pressure. With increasing air pressure, the flames contracted progressively near the nozzle exit surface, which caused the location of the generated toroidal vortices to move downward, thus the oscillation amplitude was increased as I-F = 0.14p(infinity)(0.99). (C) 2015 Elsevier Ltd. All rights reserved.