A power train of an automatic transmission realizing seven forward speeds and three reverse speeds includes an input shaft, an output gear, a transmission case, a speed reduction unit, an output unit, a first clutch, a second clutch, a third clutch, a fourth clutch, a first brake, and a second brake.The output unit includes variable input members and variably stopped members. the variable input members and variably stopped members are controlled by the clutches to realize seven forward speed and three reverse speeds at the output unit.