The action of ambient pressure variation upon buoyant laminar jet diffusion flames is studied experimentally and theoretically at low Reynolds number. For the first time, the flame height and the surface area are measured as a function of ambient pressure at different flow rates and for different burner diameters. Two distinct regimes have been found: one in which the flame flickers and the other in which it does not. For steady flames, the flame surface area increases significantly with reduction of the ambient pressure, while the height increases slightly or remains nearly constant. For flickering flames, the flame height decreases with reduction of pressure, which finally results in a decrease in the surface area of the flame. A simple theoretical model based on a cylindrical flame geometry which considers both the radial diffusion of species and the buoyancy effects, is proposed to predict the trend of surface area variation for steady flames. For steady flames, both reductions of gravity levels and ambient pressures can reduce the oxidizer flux entering the flame front, thereby increasing the flame surface area. It is shown that the local flux of oxidizer is proportional to the term (gP2)1/4. The flame flickering can be suppressed by reducing the ambient pressure. Comparisons between the effects of pressure reduction and gravity reduction upon the flame shape and the hydrodynamic behavior are also mentioned.