A hybrid vehicle comprises an engine 2 , a motor generator 4 , a torque converter 6 with a lock-up clutch 5 , and a ratio-change mechanism 7 . A control system for the hybrid vehicle comprises a rotational sensor 22 , which detects the slip ratio of the torque converter, and a hydraulic control valve 12 , which controls the engagement of the lock-up clutch. While the vehicle is moving along with the accelerator pedal being released from its stepped down condition, a driving force from the wheels is transmitted to the motor generator 4 for energy regeneration. If the slip ratio of the torque converter is equal to or smaller than a first threshold value, then only the lock-up clutch is controlled into engagement. However, if the slip ratio is between the first threshold value and a second threshold value, then additionally the motor generator is controlled in cooperative operation.