A new kinetic theory for network formation in living copolymerization of vinyl and divinyl monomers is proposed. This theory is based on the distribution of cross-linked units in the primary polymer molecule along the direction of growth. The formed cross-linked structure, such as the molecular weight distribution and the spatial distribution of cross-linked polymer chains, is investigated under the classical ring-free model. The molecular weight distribution can be multimodal if the Poisson distribution is valid for the primary polymer molecules. The radius of gyration is found to be smaller than that for the random cross-linking of linear polymer chains having the most probable distribution at the THETA state. The fractal dimension of the cross-linked polymer molecules is less than 3 except for extremely large polymer molecules, i.e., gel molecule in practice.