If a particle is not spherical, the Fraunhofer diffraction pattern is not circular, however the particle size distribution obtained from conventional particle sizer are made under the assumption of spherical particle. It is, therefore, necessary to discuss or interpret the relationship between the obtained equivalent diameter and the diffraction pattern. In this paper, this transformation problem was discussed principally for the case of ellipsoid particle. A relationship between an original size of ellipsoid and obtained equivalent particle size distribution was given in an analytical expression. The equivalent particle size clearly had double peaks in the size corresponding both to the minor and major diameter of the original ellipsoid. On the other hand, if the original particle had its own size distribution, the double peak was eliminated due to the effect of this original size distribution. In the case of random orientation of the ellipsoid in the sensing zone, the double peak was also eliminated and a rather clear single peak appeared at the minor diameter of the original ellipsoid.