A wavelength-maintaining Fabry-Perot laser diode (F-P LD) and an optical transmitter including the same are disclosed. The wavelength-maintaining Fabry-Perot laser diode comprises: a gain unit which provides a gain by a first injected current and modulates the gain; and a phase shift unit which is coupled with the gain unit in parallel, varies a wavelength for an oscillation mode of light progressed from the gain unit by a second injected current, and modulates a phase.