Pressure drop in a hydrocyclone reflects the energy necessary for a separation process. It is very important to predict pressure drop in a hydrocyclone design. In this paper, pressure drop in a light dispersion hydrocyclone is conceptually divided into two parts: dissipated pressure drop and effective pressure drop. The latter is the pressure drop in the major separation region that represents the energy converted from static to kinetic form. Based on velocity distributions established by ZHAO and MA, a theoretical model is developed to calculate effective pressure drop in the cone region of light dispersion hydrocyclones. Experimental results prove that the model can give a very good prediction of effective pressure drop. Though the calculated results are more or less higher than the measured, their differences are small enough to be neglected in a hydrocyclone design practice. It is indicated that effective pressure drop can be correlated to flowrate by Delta p(BC) = -0.000816 - 0.00186Q + 0.00667Q(2) in the range of Q = 1-5 m(3)/h at F= 6% and R-o = 4 mm for 30 mm hydrocyclone with cycloid and involute inlets. Increase of split ratio is shown to lead to decrease of effective pressure drop, while overflow orifice diameter affects the effective pressure drop in a reverse manner. According to the model developed in present paper, it is possible to study influences of operating conditions, design parameters and fluid properties on effective pressure drop. (c) 2006 Elsevier B.V. All rights reserved.