This paper experimentally studies the natural convection heat transfer characteristics on horizontal and vertical plates at pressures lower than atmospheric value. Eight different pressures ranging from 10Pa to 101.325kPa, and three heating loads were used in the experiments. It is found that natural convection heat transfer characteristics at reduced pressures are independent of the plate arrangements and present different behaviors in two pressure ranges, 10Pa p p > 1000Pa. The experimental heat transfer results are correlated into a non-dimensional equation to help the thermal design of stratosphere aircrafts and airborne equipment.