Flammability limits and flame structure of a cloud of pure stearic acid (CH3(CH2)(16)COOH) particles have been investigated under a wide range of particle diameter distributions. The stearic acid in a reservoir was heated to become liquid of 74 degrees C just above its melting point (68-71 degrees C), and sprayed by a gas-liquid two phase nozzle. The liquid droplets turned into solid particles quickly during rising along a duct to the test section. Particle diameter distributions were controlled by widely changing spraying conditions. The structures and propagating velocities of particle cloud flames under various conditions were also examined by recording the direct emission from the flames, the images caused by laser light scattering, and the schlieren images of flames. Two flammability limits were found to exist. One is the lower flammability limit which mainly depends on the mass density of smaller particles (smaller than 60 mu m in diameter). The mass density of smaller particles at the lower flammability limit is about 3.0 x 10(-5) g/cm(3). The other is the upper flammability limit which depends on the amount of all particles in the cloud. The mass density of all particles at upper flammability limit is about 34.0 x 10(-5) g/cm(3).