Provided is a wavelength tunable optical filter of a micro-electro-mechanical system (MEMS). The wavelength tunable optical filter comprises two optical fibers or optical waveguides having their optical axes aligned to each other, two lens for collimating light at leading ends of the optical fibers or optical waveguides, two or more mirrors formed on a substrate, thermal actuators supporting at least one of the mirrors, wherein one of the mirrors is actuated by thermal expansion of the actuator. Because all mirrors are formed on a substrate, a manufacturing process is simple and an initial resonance wavelength can be precisely adjusted. Since the thermal expansion is generated by the electrical current directly flowing through the thermal actuators, it can be actuated by a low consumption power. Also, since an electrostatic force is not used to move the mirrors, a sticking phenomenon between the mirrors does not occur, and the wavelength can be tunable in a wide range. Since the planar mirrors are arranged in parallel, the light alignment is easily performed, the line width is constant and the insertion loss is low.