Demands for reducing power, size, and cost of optical transponder module have been increasing. Recent development of 40Gb/s optical signal transmission technology like DQPSK modulation and sub-100nm CMOS IC technology with fT of more than 100GHz opened up new CMOS IC solution. This paper describes the prototype serializer (SER) IC for a 40Gb/s optical transponder module featuring SFI5.2 input interface and two output modes, dual 20Gb/s and single 40Gb/s, to support SONET OC-768, SDH STM 256 and ITU G.709. The data rate covers from 21.5 to 22.3Gb/s for the dual-20Gb/s mode, and from 39.8 to 44.6Gb/s for the single-40Gb/s mode.