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.