Encoder and decoder for optical code division multiple access is the key technique of the realization of optical code division multiple access communication system. A tunable serial encoder/decoder for optical code division multiple accesses is presented. Fundamental is analyzed, optimal parameters for designing it are derived, and function simulation is done. The encoder/decoder features that encoding/decoding is done serially, address codes can be tunable, encoder and decoder can be in one unit and it not only can be used in synchronous optical CDMA system but also can be used in asynchronous optical CDMA system. Therfore, it possesses some degree of practical value.