Wireless technology has become an integral part of today's life .WLAN is widely used in many conditions, especially when traditional network is difficult to install. WEP is used to ma ke wireless traffic as secure as wired network traffic .WEP suf fers from various weakness i.e. a shared key weakness, 24-bit Initialization vectors (IVs) are too short, and this puts confidenti ality at risk , poor key management etc. The CRC called cyclic redundancy check is insecure and does not prevent modification of intercepted p ackets. To overcome these problems, the existing WEP protocol is modified by replacing CRC-32 with SHA-1 hash function to enhance the security and performance of WLAN systems