To understand the inclination effects on flame spread over wood surface, a set of flame spread experiments were carried out for different sample orientation angles from -50 degrees to 20 degrees in the Hefei Plain (at the altitude of 50 m) and in the Tibetan plateau (at the altitude of 3658 m). At both altitudes, a transition zone was found at 10-20 degrees orientation for flame spread rate, the preheated length and flame angle from the horizontal. The transition zone was an external manifestation of the change of flame spread from steady state to acceleration. A new relationship of Pi(c) = 1 was established to predict the occurrence of acceleration based on theoretical analysis. Experimental data at the two altitudes suggested that the critical value of Pi(c) is about 1.1-1.2, which has a good agreement with the theoretical value. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.