A compound planetary gear set converts engine power into four forward-speeds and one reverse-speed, and the gear train provides eight forward-speeds and two reverse-speeds by utilizing the compound planetary gear set. The compound planetary gear set includes a first sun gear, first planetary gears engaged with the first sun gear, a first ring gear engaged with the first planetary gears, a second sun gear, second planetary gears engaged with the second sun gear and integrally formed with the first planetary gears, third planetary gears engaged with the second planetary gears, a second ring gear engaged with the third planetary gears, and a compound planet carrier for supporting the first to third planetary gears. The compound planetary gear set can be controlled by four frictional elements.