It is necessary to calculate and design a planetary gearbox for the work machine with power of P=10 kW and rotation speed of n=250 min-1. Considering the output power we need and the losses of the gearbox, we calculate the necessary power of electro motor and we choose the electro motor designed by Končar. Further calculation consists of the analysis of the gears by the norm ISO 6336, of stress control of input and output shaft, axle of the planetary gearbox, as well as the planet carrier, and of choosing the bearings. Analysis of the shafts, axle, planet carrier and bearings is conducted based on a hypothesis that the power is carried only by two (of three) planetary gears. During the construction of the gearbox, it is necessary to pay attention to radial dimensions of it, which have to correspond to radial dimensions of the electro motor. Since we decided that the internal gear constructs in one piece together with the housing, the dimensions of the gear are directly affecting the radial dimensions of the gearbox. Dimensions of the gear (here also the dimensions of the gearbox) change depending on a number of teeth and the module of the gear. With a proper combination of the two parameters, we achieve dimensional compatibility. Furthermore, we assemble the gearbox to the electro motor and a work machine using a flange. Here we use the same dimensions for both flanges.