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.