An encryption/decryption device and a method thereof use an RC4 algorithm to reduce a waiting time for encryption/decryption thereby avoiding data process delay. The encryption/decryption device includes a management unit, an encryption/decryption unit, and a first interface. The management unit includes a WEP seed key generator for generating a WEP seed key based on a transmitter address of first data and a cipher suite value representing a cipher protocol type for the transmitter address, an RC4 key scheduler for generating S-Box data using the WEP seed key, and an S-Box data memory storing the S-Box data generated from the RC4 key scheduler for the transmitter addresses. The encryption/decryption unit has a core for performing the RC4 algorithm corresponding to the cipher suite, encrypting/decrypting the first data using the S-Box data transmitted from the management unit, and transmitting a signal for generating the S-Box data of second data to the management unit. The first interface transmits a control signal and a data signal between the management unit and the encryption/decryption unit.