Abstract: In this papar,the features of theoretical meshing of rotor with stator and its behaviors under operation of screw pair with different lobe ratios are analysed.It is considered that the differential pressure of a screw motor or pump is equal to the sum of differential pressure at the common screw meshing lines of all closed up chambers.It has been established through the study of hydraulic transverse forces that the conversion of the Equid pressure energy and the mechanical energy in the operating single-rotor system with different lobe ratios follows the law of conservation of energy.The form-ula of average torque given in this paper is based on the rational choice of differential pressure at the common screw meshing of closed chamber,including all the space-geometrical parameters.Some new concepts with respect to the mome-ntary equivalent meshing length of rotor with stator etc,for evaluating the influence of space-geometrical parameters are put forward.The feature of the two L^p/2 intervals of i>1/2 single-rotor systems that causes slanting torque M_s is discussed.The slant of rotor under operation of screw pair is believed to be resulted from all the transversal forces applied to the rotor.