To reveal clearly the motion behaviors and velocity distributions of the unburned particles ahead of flame front in hexadecanol dust explosion, the flame propagation process was recorded by a high speed camera combining the particle image velocimetry (PIV) techniques. Flame region was recognized from the direct-light-emission photographs by the color model; Otsu's automatic threshold segmentation model and Fast Fourier Transform were adopted to set the threshold and to measure the flow field, respectively. It was observed that the propagating flame consisted of blue-spots of flame at the leading zone and luminous flame behind them. The flame firstly propagated towards the small particles nearby, when the small particles were completely pyrolyzed, the local pre-mixing flame would continue to heat the larger particles, establishing the local diffusion flame, and then the isolated blue luminous spots. Single dust particle velocity changed with the distance to the flame front. There existed a stagnation region, in which the particle velocities approached zero. Beyond this region, the particles settled down at a constant velocity. Vortex structures appeared in the regions where the settlement particles encountered with the outward movement particles. (C) 2014 Elsevier B.V. All rights reserved.