A transmission for a tractor has a forward/reverse transmission unit for selectively changing a rotational direction of power of a first shaft connected with an engine and transmitting the power to a second shaft connected with an axle shaft, a main transmission unit installed between the second shaft and the axle shaft and having a plurality of gears to speed-change the power transmitted from the second shaft in four-speed, and a sub-transmission unit installed between the main transmission unit and the axle shaft and having a plurality of gears to speed-change again the power transmitted from the main transmission unit in four-speed. The transmission realizes forward/reverse 16-speed while having a compact structure.