The invention relates to a control system of a torque tool (1) provided for tightening and unscrewing of screw connections with adjustable torque. The system has a controller (3) of an electric motor (2) of the torque tool (1), and comprises a sensor (6) measuring the torque. The sensor (6) is configured to measure torsional deformations of a reduction gear (5) component of the torque tool (1) transferring the torque. Moreover, the sensor (6) is coupled with the controller (3), and the controller is configured to correct the output control signal to the electric motor (2) on the basis of the signal from the sensor (6) of the measured actual torque. The application also refers to a method of controlling the torque tool (1).