A random sampling technique in which polymer molecules are sampled from an infinite number of polymer molecules in the reaction mixture is used to predict the molecular weight distribution (MWD) development during free-radical polymerization that involves polyfunctional chain transfer agents (P-CTAs). For an ideal ease where the chain transfer constant is unity and chain stoppage is dominated by chain transfer reactions, the analytical solutions for the full MWD as well as the average molecular weights can be derived in a straightforward manner. For more complex reaction systems where nonrandom history-dependent kinetics is important due to bimolecular termination reactions and the substitution effect of the P-CTAs, the Monte Carlo simulation on the basis of the random sampling technique enables one to estimate the statistical property development quite effectively. When bimolecular termination reactions and/or the substitution effects are involved, the MWD does not necessarily become narrower with polymerization in contrast with the ideal case. The decreased reactivity of the functional groups in a P-CTA due to the substitution effect may increase or decrease the average molecular weights depending on the magnitude of the chain transfer constant, concentration of P-CTA, the mode of bimolecular termination (disproportionation or combination), and the degree of the substitution effect.