A wavelength tunable laser light source necessary for a Wavelength Division Multiplexing (WDM)-based optical communication system, and more particularly, a wavelength tunable laser light source for maintaining a stable wavelength without a wavelength locker is provided. The wavelength tunable laser includes an optical fiber and a semiconductor device. The optical fiber can implement multiple reflection peaks. The semiconductor device includes a mode size converter section, a gain section and a DBR mirror section. One facet of the semiconductor device, which is adjacent to the optical fiber, has an antireflection coating layer. Therefore, the wavelength tunable laser based on an optical fiber containing multiple reflection peaks can maintain excellent wavelength stability insensitive to current injection, temperature, and environments without a wavelength locker.