The unprecedented 1,3-dipolar cycloaddition of 1,3,5-triazinanes and aziridines has been described. With readily available starting material, this method offers efficient access to a wide range of functionalized imidazolidine derivatives in moderate to good yields (up to 92%) under mild conditions. Preliminary mechanistic investigations show that the ring opened zwitterionic pathway product dominated over the second-order nucleophilic substitution-like product. This protocol is very promising because the reaction is scalable, and gives versatile transformation of the products into other functionalized imidazolidines.