A new algorithm of Monte Carlo (MC) simulation has been developed for network formation by a kinetic approach. This method is very general and can be directly applied to various systems with a complex chemical reaction mechanism. The strategy of the MC method is divided into two steps, selecting the type of the reaction mechanism and then choosing the reacting molecules. This makes it possible to accomplish the simulation in a reasonable time. Then the systems of epoxy resins cured with primary amines with a substitution effect and etherification were analyzed by this method. The changes of the structure, such as the molecular weight distribution of polymers, gel fraction, and cycle rank, were calculated, and the gel point was determined by the method of reduced average molecular weight. It is found that, if the effect of topology is not taken into account, very few times of the intramolecular reactions occur before gelation for a large finite system. The gel point and the average molecular weight profiles simulated are well consistent with those by another kinetic approach and experimental data, and the changes of the molecular weight distribution (MWD) depend on the rate of etherification.