Reliability and maintenance analysis of hydraulic positive machines basicly focused on the processes of their wear and failure. But in order to correctly assess the mechanical and volumetric efficiency of a hydraulic motor, both at the stage of development research or at the stage of control tests during its exploitation, the working volume of this motor must be correctly determined. Therefore this paper proposes a new method of assessment of the size of the working volume of a hydraulic motor. It has been shown that the hydraulic motor absorbency per one revolution of this motor shaft is a non-linear function of pressure drop in working mechanism of the motor and non-linear function of motor rotational speed. Thus the relation between the working volume of a hydraulic motor and the pressure drop in the motor working mechanism is a non-linear function. This working volume as a funcition of pressure drop has been called the actual working volume. The correctness of the proposed method was confirmed experimentally