Nonfilled triallyl isocyanurate (TAIC) cured resin obtained directly by polymerization of TAIC monomer was too brittle for practical use. The brittleness of cross-linked resins has been thought to be due to the inhomogeneity of their network structures, which consist of colloidal particles as evidenced by electron microscopy. However, the complete loss of flexibility of poly(TAIC) chain does not satisfy the prerequisite of locally enhanced occurrence of intramolecular cross-linking reaction inducing microgelation to form a colloidal particle. Therefore, the correlation between brittleness and network structure of TAIC resins was further discussed, especially focusing on the characterization of resulting TAIC network polymer precursors (NPPs) by size-exclusion chromatography (SEC)-multiangle laser light scattering (MALLS) -viscometry since the ordinary explanation for the brittleness of cross-linked resin does not appear to be the true explanation for TAIC resin's extreme brittleness. Thus, an alternative explanation for the brittleness of TAIC resins is provided: insufficient growth of the network structure of TAIC resin because of steric hindrance of the cross-linking reaction between sterically crowded growing polymer radical and pendant allyl groups belonging to the rigid primary polymer chain. This imperfection of cross-linking, providing not only insufficient network structure formation but also incorporation of abundant dangling chains into network polymer, could be one of the principal causes of brittleness, even for common cross-linked resins.