Abstract: It is very necessary to know the effects of various parameters on pressure drop in optimum design of a hydrocyclone.Based on the fluid flow inside light dispersion hydrocyclones,an analytical model is established to calculate pressure drop in the conical zone of hydrocyclones.Analyses show that the calculated pressure drop in this zone represents the“effective”pressure drop necessary for conversion of static energy to dynamic energy in the separation zone,and that the one in the inlet area determines the pressure loss for achieving changes in direction and cross-section area of fluid flow bundle.It is reasonable to say that higher“effective”pressure drop ratio indicates more energy effectively consumed in the separation of two-phase liquid inside hydrocyclones.Comparisons of calculated pressure drop to the measured reveal that this pressure drop ratio is approximately constant for a certain hydrocyclone when fluid properties keep the same.However,it will be significantly changed if structural parameters or fluid properties change.