A convenient one-step synthesis of five-membered or six-membered imino-protected cyclic guanidine via an intramolecular ring-closure reaction of alkyl diamine (2a-2g) with 1, 3-diamino-protected methylisothiourea (la and 1b) was established and investigated. Amino guanidine such as 3-(2-aminoethyl)-l, 2-dibenzyloxycarbonylguanidine (4a) has been proved to be the intermediate of the reaction via utilizing mono-protected diamine as starting material. The intramolecular ring closure of 4a results in 2-benzyloxycarbonyliminoimidazolidine (3a). This new one-step synthesis has advantages of simple condition, easy workup procedure and reasonable yield.