To reveal the effects of particle size distributions on flame propagation mechanism during octadecanol dust explosions clearly, the flame propagation through dust clouds with three different particle size distributions in an open space were recorded using an approach combined by high-speed photography and band-pass filter. From the direct light emission photographs and CH emission photographs, it has been demonstrated in the experiments that the flame front structures were changed as varying the particle size distributions. Flame propagated in the dust cloud with a smaller particle size was characterized by a regular shape and spatially continuous combustion zone structure, which was similar to the premixed gas explosions. On the contrary, when flame propagated through the dust cloud with a larger particle size, discrete blue luminous spots appeared surrounding the yellow luminous zone. Further increase in diameter resulted mostly in the appearance of blue spot flames and the disappearance of yellow luminous zone in the flame propagation process. Additionally, a simple mathematical model was developed to determine the critical particle size to illustrate the different flame propagation mechanisms in octadecanol dust explosions. (C) 2013 Elsevier B.V. All rights reserved.