A reduction gear includes a case including a hollow cylinder having teeth on its inner wall; a ring gear having outer teeth meshed with the teeth of the cylinder, and inner teeth; and an eccentric gear assembly mounted in the cylinder and including a hollow eccentric shaft including rear and front eccentric sections, and first and second gears rotatably secured onto the rear and front eccentric sections respectively. Rotating an input shaft in a first speed will rotate the eccentric shaft to rotate the first and second gears around the inner teeth of the ring gear eccentrically, thereby rotating the output shaft in a second speed lower than the first speed. The invention has an increased number of meshed teeth between each of the first and second gears and the ring gear so as to distribute a radial load to more teeth in rotation.