A new algorithm of Monte Carlo (MC) simulation has been proposed for the imidazole/epoxy/phenol resin system. The strategy of the MC method is divided into two steps: the first is deciding the type of reaction mechanism, and then choosing the reacting functional groups. The structural properties corresponding to the conversion change, such as the average molecular weight, the molecular weight distribution, the sol-gel fraction, and the branch point distribution of the sol part can be simulated by this method. The BPA molar ratio seems the most important factor in this system. Because it dominates the final structure of the cured resins, either linear or branched. The theoretical results were compared and verified by the gel permeation chromatography and the sol extraction analysis. Both theoretical and experimental results coincide with each other very successfully.