Determination of tractor moments of inertia is not so easy. There are known few methods of indirect measurement of moment of inertia, but the hanging tractor is necessary in this methods. Suspension of the tractor is a long shot. This is the reason, that most of authors assume the quantity of moment of inertia only intuitionally, without measurements, in their computation. Tractor moments of inertia can be evaluated through "geometric-mass " tractor model. Main tractor elements, like corps, wheels, cabin, wheel reduction gears, half-shafts, can be admitted by non complicated geometric body, like cylinder, ring, cone, rectangular, torus, plate, box constructed from plate. Dimensions and weightiness of this geometric body must be the same that tractor units. The main moments of inertia of this solid are easily calculated. Steiner principle provide calculation main moments of inertia of all tractor body. These moments of inertia were applied in mathematical models of the tractor, built for eigenfrequencies calculation. Tractor vibrations were measured for verification of the models. These measured frequencies were similar to frequencies calculated from received models. It means, that suggested method of tractor moments of inertia determination, is accurate. This easy method allows to leave out big mistakes in calculation of tractor free vibrations frequencies.