As a part of a research program for pharmacologically interacting sym‐triazine derivatives with hexahydrotriazine ring, novel hexahydrotriazine derivatives (1−13) were successfully synthesized. The synthesis of hexahydrotriazines was carried out from the assembly of three molecules of various amines and three molecules of formaldehyde by “1 + 1 + 1 + 1 + 1 + 1” cycloaddition. The synthesis mechanism is probably passes through imines, which trimerizes to give the hexahydrotriazine ring. All synthesized hexahydrotriazines (1–13) were screened for their cytotoxic activity by cancer cells. As a result, compound 1–9 reduced the viability of T98G cells in a concentration‐dependent manner. At a concentration of 100 μM, compound 9 exhibited the greatest cytotoxicity at 94.6%. The reason is that compound 9 is C5H5N that has a resonance hybrid structure and exhibits aromaticity. It is considered to be active because it has a structure that is a raw material for pyridine derivatives as mainly medicines and pesticides.