An eight-speed automatic transmission for a vehicle may include a single pinion planetary gear set, a double pinion planetary gear set, a compound planetary gear set, a first clutch, a second clutch, a third clutch, a fourth clutch, a first brake, a second brake, and a one-way clutch. The first and third clutches are disposed in series or in parallel between the compound planetary gear set and the output shaft. A first clutch piston chamber for operating the first clutch is disposed between the third clutch internal diameter and the input shaft. A third clutch piston chamber for operating the third clutch is disposed between the first clutch piston chamber and the output shaft. The second and fourth clutches are disposed between the compound planetary gear set and the double pinion planetary gear set. Thus, the overall volume of the automatic transmission may be reduced.