Abstract: Smart cards are secure, compact and intelligent data carriers, which can offers strong authentication and guaranteed non- repudiation. With the traditional authentication solution, the static password is rarely altered and maintained in the verifier table on the server. This is bringing forth the important attacks of replay attacks, guessing attacks, modication attacks, and stolen-verier attacks. However, the dynamic ID-based authentication solution can solve the problem. I-En Liao et al proposed a dynamic ID-based user authentication scheme using smart cards. The scheme has a lot of advantages, such as avoiding a variety of attacks, mutual authentication and no verifier table, but it can′t resist to stolen attack. A improved scheme was presented to remedy their weaknesses. Compare with Liao-Lee-Hwang′s scheme, the improved scheme not only avoid stolen attacks but also reduce the computational costs.