The validation of particle size analysis by laser diffraction (LD) method should be done using reference particles, whose size distribution is better to have a range over one decade of size, according to ISO 13320-1. Two kinds of samples met this request, 1-10 mu m and 10-100 mu m samples, were distributed from the Association of Powder Process Industry and Engineering, Japan (APPIE). In this paper, the results of the round robin test of these two samples by LD method were reported together with the comparison of the size distributions measured by electrical sensing zone and scanning electron microscope methods. 1-10 mu m sample was well dispersed in water without detergent, but a few drops of detergent sometimes needed for dispersing 10-100 mu m sample. For 1-10 mu m sample, the mean particle size by LD methods was slightly smaller than that by SEM method, and was agreed with each other results by seven different LD instruments. For 10-100 mu m sample, the mean particle size by LD methods agree well with that by SEM method, but the discrepancy of one instrument in larger size range became larger than the results of other instruments.