A method is presented, based on the probability model proposed by Macosko and Miller, to calculate all the average molecular weights of nonlinear polymers. ($) over bar M(W), ($) over bar M(z), ($) over bar M(z+1), and higher average molecular weights can be determined as a function of reaction extent directly without the calculation of the whole distribution. The results indicate that this approach can be easily formulated and applied for many nonlinear polymerization reactions.