An actuator for adjusting a seat includes a motor, a gear-box, and an output member. The gear-box includes a casing accommodating a two stage gear train connecting the motor to the output member. The gear train has a first and second transmission mechanisms. The first transmission mechanism includes a worm on a shaft of the motor, and a worm wheel meshed with the worm. The second transmission mechanism is a movable teeth gear transmission mechanism and has an eccentric wheel that rotates with the worm wheel, an internally toothed ring gear fixed to the casing, a fluted disc with radial grooves formed in its outer periphery, and rollers mounted in the radial grooves. The eccentric wheel drives the rollers to mesh with the ring gear to rotate the fluted disc, to drive the output member.