A new authentication scheme based on a one-way hash function and Diffie-Hellman key exchange using smart card was proposed by Yoon et al.in 2005.They claimed that the proposed protocol is against password guessing attack.In this paper,the author demonstrate that Yoon’s scheme is vulnerable to the off-line password guessing attack by using a stolen smart card and the DoS attack by computational load at the remote system.An improvement of Yoon’s scheme to resist the above attacks is also proposed. A new authentication scheme based on a one-way hash function and Diffie-Hellman key exchange using smart card was proposed by Yoon et al. In 2005. They claimed that the proposed protocol is against password guessing attack. In this paper, the authorizer that Yoon’s scheme is vulnerable to the off-line password guessing attack by using a stolen smart card and the DoS attack by computational load at the remote system. Improvement of Yoon’s scheme to resist the above attacks is also proposed.