A powertrain system includes an internal combustion engine, a motor generator and a control device. The motor generator includes a rotating shaft connected to a crankshaft of the internal combustion engine via a torsional damper. The powertrain system is configured such that the crankshaft and the above-described rotating shaft are not connected to a drive shaft of a vehicle at least at the time of engine start. The control device is configured to execute a cranking torque amplification control that controls the motor generator such that the MG torque output from the motor generator for cranking the internal combustion engine fluctuates in a resonant period of the torsional damper while making a fluctuation center of the MG torque higher than zero.