In this article, the influence of pressure variation due to altitude change on flames is studied. A series of largely buoyancy-driven acetylene jet fire experiments was conducted at two places in China, namely Lhasa and Hefei, with altitudes of 3658m and 50 m, respectively. A range of parameters, including flame height, flame and plume centerline temperatures, transmittance through smoke, and irradiance were measured and compared. The flame height at low altitude is higher than that at high altitude. The centerline temperatures of the flame and plume regions exhibit opposite variations with pressure. The soot mass concentration resolved by transmittance measurement follows a weaker power-law dependence on pressure under substandard atmospheric pressure than under pressure greater than standard atmospheric pressure. The radiation heat flux produced from flames of the same burning rate displays a positive proportionality to pressure.