Following our earlier study on the behavior of lycopodium dust flames, further experiments using a particle image velocimetry system with a high-resolution video camera have been conducted to clarify the mechanisms of laminar dust flame propagation in a vertical duct. Lycopodium, a nearly equal-sized particle, has been recognized as being monodispersed, but it was found that an actual lycopodium dust cloud consisted of individual and agglomerated particles. Corresponding to the particle forms, the reaction zone showed a double flame structure, consisting of enveloped diffusion flames (spot flame) of individual particles and diffusion flames (independent flame) surrounding some particles. Due to the convective flow caused by a flame, part of the gravitational settling particles was shifted to the surrounding sides and the rest of the particles changed their movements to upwards in front of the flame. Such particle movement causes a dynamic variation in dust concentration ahead of the flame, which propagates at lower dust concentration rather than the mean concentration. Although the flame moved discontinuously on a micro scale, an overall constant flame velocity was found, presumably due to the dynamic variation in dust concentration and induced flow ahead of the flame. Judging from the above-mentioned movement of single particles in front of the flame, a residence time of the unburnt particle in the preheating zone is needed to form combustible gases close to the particle. This residence time depends on the preheating zone thickness, the particle velocity and the flame propagation velocity. The observation of the movement of a single particle suggested a flame propagation mechanism where an enveloped and diffusion lycopodium dust flame discontinuously propagates from one particle to those adjacent in a laminar suspension. (C) 2001 Elsevier Science Ltd. All rights reserved.