Cycloidal gear design is based on forming geometry and material features of load carrying assembly to fulfil the following criteria of the construction: functionality, reliability and life. Life of cycloidal gear is determined by life of the weakest kinematic pair in power transmission system. That system consists of the following, connected in series, rolling pairs: the central cylindrical roller bearing, the set of rolling bolts in straight-line mechanism and the special meshing of planetary wheel. Dimensions of the gear are determined by combination of three basic parameters: curtailment coefficient of epicycloid, gear eccentric and roller diameter of co-operating wheel. The combination mentioned above should fulfil conditions of internal cycloidal meshing and assure proper reliability in power transmission system. In the paper there are presented results of calculations of fatigue life of rolling pairs in cycloidal gear for different parameters affected gear overall dimensions. These results made possible to find optimal geometry of the gear in fatigue life aspect.