An improved structure of gear box for the cutter of cultivators has a box composed of a left cover and a right cover. The inside of the box has an input shaft having a sprocket gear, a transmission mechanism, a left outer output shaft is composed of a gear and a shaft sleeve having a through hole, a right outer output shaft is composed of a gear and a shaft sleeve having a through hole and an inner output shaft having a sprocket gear. The transmission mechanism is composed of a first chain, a first turning axle, a second turning axle and a second chain. The first turning axle has a sprocket gear and a driving gear. The second turning axle has a sprocket gear, a driven gear, a left gear and a right gear. The sprocket gear on the first turning axle is connected with the sprocket gear on the input shaft by the first chain. The driving gear on the first turning axle is meshed with the driven gear on the second turning axle. The sprocket gear on the second turning axle is connected with the sprocket gear on the inner output shaft by the second chain. The left gear is meshed with the gear on the shaft sleeve of the left outer output shaft. The right gear is meshed with the gear on the shaft sleeve of the right outer output shaft. The inner output shaft penetrates the through hole of the shaft sleeve of the left outer output shaft and the right outer output shaft. Thus, a clockwise rotating left outer output shaft and a clockwise rotating right outer output shaft and an anti-clockwise rotating inner output shaft in a same axis are provided.
