In this paper a distribution function of polymerization degree is presented. This distribution function contains three parameters a

b and P0. When a equals 1 the distribution function becomes schulz distribution. Because the distribution function contains three parameters it possesses broad applicability. The distribution function has been tested for eleven kinds of polymers. It is found that the function proposed by the present authors provides good agreement with experimental data. Analytical expressions of different average polymerization degree have been obtained.