Portable communication systems can provide mobile users with global roaming services. Recently, Youn and Lim proposed a delegation-based authentication protocol which achieves unlinkability for secure roaming services. This paper indicates that there are two drawbacks in Youn and Lim’s protocol: 1) the synchronization problem will lead to a fail in on-line authentication; and 2) the exhaustive search puts a heavy burden on the off-line authentication process. Moreover, based on Youn and Lim’s protocol, a remedy is proposed to address these problems. It is worthwhile to note that the proposed remedy not only keeps the original advantages but also enhances the security and performance. Recently, Youn and Lim proposed a delegation-based authentication protocol which achieves unlinkability for secure roaming services. This paper indicates that there are two drawbacks in Youn and Lim’s protocol: 1) the synchronization problem will lead to a fail in on-line authentication; and 2) the exhaustive search puts a heavy burden on the off-line authentication process. Moreover, based on Youn and Lim’s protocol, a remedy is proposed to address these problems. It is worthwhile to note that the proposed remedy not only keeps the original advantages but also enhances the security and performance.