A power train apparatus may include an engine including a crankshaft and an engine block, a motor housing connected to the engine block and of which a housing opening is formed, a rotor portion connected to the crankshaft through the housing opening, of which a magnet is connected to a first side thereof and the rotor portion to which a balance portion is formed for compensating for imbalance rotational energy transmitted from the crankshaft; a stator portion disposed between the rotor portion and the motor housing and connected to the motor housing and a transmission connected to the engine block.