Abstract: Mobile agent systems, which have several advantages and raise the whole networks efficiency by using asynchronous communication mode as software agents, play important roles in distributed computer systems in the future and find wide applications. However, the mobile software agents are vulnerable to attack from malicious programs running on the hosts, and it is the key to the mobile agents’ security running to protect them against being tampered by malicious hosts. It is necessary to develop new secure protocols that can be used in mobile agents because the old ones are unsuitable to these environments. Combined with an onion routing scheme, a mobile agent system, by using the cipher text only signature verification, is presented to enhance its security and reliability. Finally, analysis of its security is conducted.