An optical transmitter is provided which optimizes DC bias voltage input to an optical modulator employing a duo-binary modulation scheme. The optical transmitter including a signal combiner which converts an electrical signal for optical transmission into a high-speed electrical signal, and an optical modulator which receives and modulates the high-speed electrical signal from the signal combiner further includes a frequency divider to divide frequency of a sinusoidal watchdog clock signal from the signal combiner and to output the divided signal to be added to direct current (DC) bias voltage input to the optical modulator; and a bias voltage modifier to adjust the DC bias voltage by analyzing the modulated electrical signal from the optical modulator. Accordingly, it is possible to conveniently optimize the DC bias voltage with a simple design.