PURPOSE: To provide a power transmission connected to a motor/generator and ensuring the optimum operation of an engine, and a power train therewith.

CONSTITUTION: This power transmission 10 disposed between the engine 11 and a drive ratio selection transmission 39 of a vehicle can be selectively- connected to a motor/generator 14, adds and subtracts torque generated by the motor/generator to the torque generated by the engine, interconnects the engine and the motor/generator to the drive ration selection transmission, and employs compound first and second planetary gear sets 19, 21. A common gear member 16 serves as the ring gear for the first planetary gear set and the sun gear for the second planetary gear set. The power transmission provides advantages in operation which had not been attained with conventional devices, thus it is possible to operate the engine with desirable engine emissions.