A hydrostatic-mechanical gear unit wherein input power is divided into an hydraulic power branch and a mechanical power branch. The hydraulic branch includes two hydrostatic units in tandem. Through gearing the hydraulic branch is directed to a first internal gear for transmitting the rotational hydraulic power. The mechanical branch through gearing leads to a second internal gear for transmitting rotational mechanical power. The internal gears together with pinions form part of a planetary wheel set for integrating the mechanical power and the hydraulic power. An intermediate shaft connected to the pinion carrier conveys power to an output shaft through a coupling in a first forward driving stage and in the reverse driving stage, and a coaxial shaft connected with a sun wheel conveys power to the output shaft through a coupling in a second forward driving stage.