A power train for eight forward and one reverse speeds may include input shaft, output gear, transmission case, speed shift unit, output unit, four clutches and two brakes. The speed shift unit includes a first operational member fixedly connected to the input shaft, a second operational member selectively coupled to the input shaft; a third operational member fixedly connected to the transmission case; a fourth operational member selectively coupled to the sixth operation member; and a fifth operational member selectively coupled to the fourth operational member. The output unit includes a sixth operational member fixedly connected to the fifth operational member, a seventh operational member selectively coupled to the input shaft and the transmission case, an eighth operational member selectively coupled to the input shaft, and a ninth operational member connecting the output gear. The four clutches and two brakes control pertinent operation members to change the rotational speed.