A mechanical-hydraulic transmission gear and a method of controlling the same which are used for construction vehicles or machines, and which are designed in such a manner that, when a motor is rotated at a high speed, the transmission of the power is carried out via a mechanical transmission gear alone to control a hydraulic power loss to substantially zero, and in such a manner that, when the motor is rotated at a low speed, the transmission of the power is carried out via a hydraulic transmission gear alone to make easier a speed control operation and a forward and reverse rotation switching control operation. This mechanical-hydraulic transmission gear includes a hydraulic transmission gear (10) having a hydraulic pump (11) and a hydraulic motor (13), a mechanical driving gear (30), a forward and backward movement change-over means (26), a speed control means (27), a means (22) for detecting the rotational speed of an output shaft (14), and a control means (20) adapted to compare the signals from the forward and backward movement change-over means, speed control means and rotational speed detecting means and carry out the controlling of the connection and disconnection of a clutch (31) for an input shaft (2) in the mechanical transmission gear and the controlling of the capacities of the pump and the hydraulic motor. The method of controlling the mechanical-hydraulic transmission gear includes the steps of disconnecting the clutch for the mechanical transmission gear when the rotational speed of the output shaft is lower than a predetermined level, to transmit the power by the hydraulic transmission gear alone, and connecting the clutch for the mechanical transmission gear when the rotational speed of the output shaft is higher than the predetermined level, to transmit the power by the mechanical transmission gear alone, and minimize in accordance with a command from the control means the power absorbed into the hydraulic transmission.