—We propose a security authentication policy, Dynamic Session Key Policy (DSKP), for a secure handoff between the UMTS and IEEE 802.11 WLAN. This policy is founded and improved from Dynamic Key Exchange Protocol (DKEP). It redeems the confidentiality of the communication association using the asymmetric and symmetric encryption. The one time password system is used for the key exchange of the sessions. The transition of the communication states in DSKP are seamless and cannot be personated. From the security analysis, using the DSKP can avoid possible attack in wireless circumstance. EAP-SIM and EAP-AKA, commonly used for authentication protocol in present WLAN, were also studied to compare with DSKP. From our security analysis, DSKP shows better security grades. Keywords- security authentication; WLAN; UMTS