In the field of ceramics, the particle size distribution of submicron powders has been measured by using many different types of equipment, which are based on different measuring principles. However, at present, the laser diffraction and scattering method is the most popular method in particle size analysis. Although this method is good for operation, the refractive index of the particles is required in order to calculate the size distribution. Proper results will not be obtained if the refractive index is not considered in the calculation. For the measurement of an unknown sample, this is the weakest point of the laser diffraction and scattering method. In this study, an estimation method of the optimum refractive index was established by simple data arrangement and the accuracy of the results was improved.