A powertrain apparatus includes a motor connected to an input shaft, a first planetary gear train, which is disposed to be coaxial with the motor and which includes a first rotation element, a second rotation element, and a third rotation element, a second planetary gear train, which is disposed to be coaxial with the first planetary gear train and which includes a fourth rotation element, a fifth rotation element, a sixth rotation element, and a step pinion, and a differential gear, which is disposed to be coaxial with the second planetary gear train.