Abstract: Electronic commerce is a main trend in Internet applications, which should be based on fair exchange. A new fair exchange protocol with off line semi trusted third party is proposed. It is also efficient in the sense that it only needs 4 messages in a transaction. In the meantime all messages are encrypted naturally, which makes it appealing over large open networks that have little or no physical security, such as Internet. It is based on publicly verifiable secret sharing （PVSS） theory.