The effect of the opening size distribution of test sieves on the separation characteristics was investigated. An estimation equation for the separation size taking into account the sieve opening size distribution was derived on the basis of the probability of particle passage through a sieve opening. An experiment was carried out with three test sieves, which had the same nominal opening size of 250 microns but different opening size distributions, and four types of alumina abrasive powder as the samples. The estimated separation size of test sieves agreed well with the experimental one. The separation size increased as the opening size distribution became broader. The effect of the sieve opening size distribution on the separation characteristics is discussed by the estimation equation. The required accuracy of openings for precision particle size measurement is within approximately 3% of the coefficient of variation. Regarding the sharpness of separation, sieves with a broader opening size distribution exhibit indistinct separation. A longer sieving time leads to larger separation size and indistinct separation. Consequently, it is clarified that there is an optimum sieving time for particle size measurement by sieving.