A high-speed wavelength selectable optical source and a method thereof are disclosed. The wavelength selectable optical source includes at least one optical source having a multimode oscillation spectrum, a demultiplexer and an optical switch for extracting light having a certain selected wavelength. A certain wavelength can be selected among the multimode wavelengths using the demultiplexer and the optical switch, and thus the wavelength can discontinuously be tuned at high speed. Also, by making the light having the selected wavelength incident to the multimode optical source, the power of the output light can uniformly be maintained and maximized through injection locking.