It is pointed out that the conclusion in the reference [1] is not correct. The formula for Fraunhofer diffraction, which describes the diffraction pattern on an observing screen locating at the divergence center of an illuminating spherical wave and being parallel with the diffracting screen, is deduced from the Rayleigh-Sommerfeld formula of the scalar diffraction theory. The application condition of the formula is also discussed. Applying the formula to the particles scattered randomly over a 2-D plate, the light intensity formula and the measurement formula for a particle diameter, which describe the Fraunhofer diffraction pattern of the particles, are derived. The authors demonstrated that, when the Fraunhofer diffraction method is used to measure the average dimension of the particles, it is necessary to adopt the formula for the case of a big diffraction angle, the related correct result can be obtained. There is not any systematic error mentioned in reference [1] that increases with the value of diffractionangle.On the contrary,under the condition that the diffracting distance doesn't change,the measurement accuracy increases and the accidental error decreases as the diffraction abgle increases.