Abstract: In this paper, based on the theories of light scattering by small particles, a FAM laser particle sizer is developed using both Fraunhofer diffraction and Mie scattering theory. After discussing the working principle, the measurement results for various powder particle samples and droplets are presented. The tests show that the FAM laser particle sizer has advantages of high accuracy, wide application range and is suitable to the size measurements of powders and droplets in two-pbase mediums.