In measuring the particle-size distribution by small angle X-ray scattering the particle form must be known.In general the form of particles considered to be spherical ,but in practice, the form of particles was very complexity.We statistcally separate the forms of particles into sphere-like; flat-like and rod-like ones. The general method for calculating the form scattering function has been given, and we have calculated the form scattering function for sphere-like particles, and given an adjustable parameter δ that can be used in general analysis.