Abstract: Recently, Yi et al..(2002) proposed an ID-based key agreement protocol for multimedia encryption. The protocol was built on both the Diffie-Hellman key exchange protocol and the RSA public key cryptosystem. Yi et al. analyzed the security of the protocol, and understood that the protocol is robust to the malicious attacks. However, this paper shows that the protocol is vulnerable to certain malicious attacks such as forging secret information and tampering exchanging messages and analyzes the reasons that the protocol suffers these attacks. This paper points out that it may be hard to improve the protocol due to the inherent flaw of the protocol.