A power train of an automatic transmission attains eight forward speeds and one reverse speed. The power train includes first, second, and third planetary gear sets, four clutches, and two brakes. The first planetary gear set generates a reduced rotational speed, the second planetary gear set generates a reverse rotational speed, and the third planetary gear set generates the eight forward speeds and the one reverse speed by using the reduced rotational speed, the reverse rotational speed, and a rotational speed of an input shaft.