In the nonlinear polymerizations, the equilibrium number fraction distribution may have a commonly invariant property. This invariant property is revealed by taking the A(alpha 1), ..., A(alpha s) - B-b1, ..., B-bt-type reaction proposed by Stockmayer as an example to yield an outcome that under the transformation between the variables for describing the total system and the variables for describing the sol, the distribution is left invariant. This invariant property may lead to the fact that the average polymer quantities for the postgel can be easily obtained from the corresponding quantities for the pregel via a simple replacement of the variables for the pregel by the sol variables for the postgel. For illustration, the kth radius of the A(alpha 1), ..., A(alpha s), - B-b1 ..., B-bt-type polymerization is discussed.