The motion of solid particles within a hydrocyclone is analysed by solving dynamical equations in two dimensions, and with the aid of these equations the method of calculating the fractional recovery of particles is presented. Separation data by various previous experiments are in satisfactory agreement with the theoretical curves obtained by this method. Moreover the characteristic equations for cyclone as well as the formula for imaginary separation size are also presented. On the basis of these equations, effects of the relative size of inlet nozzle to cyclone diameter, and of the form of its cross-section on the accuracy of classification are discussed. Finally collection efficiency and thickening ratio are calculated by the use of fractional recovery already obtained. These theoretical values seem to confirm the experimental data.