Smoke is the important parameter of fire detection, and smoke particle size and its distribution may affect the sensitivity of smoke detectors directly. However, up to now there had no report about such research. In this paper, the theory, configuration and measurement method of Scanning Mobility Particle Sizer (SMPS) were described and the particle size and its distribution of four standard fire smoke were measured by SMPS. The particle size distribution function of smoke were studied by mathematic fitting methods, and the result of research showed that Logarithm Normal Distribution (LND) could represent the distribution of fire smoke particle size. The results may be benefit for the foundational research on fire smoke detection.