An apparatus and a method for encrypting and decrypting data in a wireless LAN includes additional blocks for generating key sequence by a secret key and generating key sequence by ICV in order to solve disadvantages of conventional WEP apparatus and method for encrypting/decrypting data suggested by IEEE 802.11, and the key sequence by the ICV is dual-encrypted by XOR operated with the transmission data, and thereby, wiretapping and sensing from outer side can be blocked.