Owing to the broad measur able particle size range, wide applicability, reliable measurement and good reproducibility, Fraunhofer Diffraction based laser particle sizers have recently found growing uses in different industrial areas. Nevertheless, the accuracy of measurements made in small particle size range is not sufficient. In this paper the theoretical limitations of different Fraunhofer Diffraction based particle sizers are discussed and a combined Fraunhofer Diffraction and Mie Scattering(FAM)based laser particle sizer is developed, Comparative measurements show that the performances of FAM are much better than those of the Fraunhofer Diffraction based particle sizers.