This paper examines the security vulnerability, demanded service, layering consideration for local area networks(LANs), and proposes a new key distribution system suitable to local area networks. the new key distribution system Is applicable to time quantum in encrytion key and then the time used arid the quantity of encryption message using the encryption key are limited. A system suggested in this paper can achive some higher concurrency of key and authentication of the opposite party. Even through a encryption key expose by reason of wiretapping of a frame unit, It has achived some more security and safety, because the contens of all traffic messages are not known.