Three-dimensional　laminar　fluid　flow　and　heat　transfer　in　a　micro　heat　sink　with　cavities　and　internal　ribs　are　investigated　using　numerical　methods.　Moreover,　according　to　the　second　law　of　thermodynamics,　the　model　of　entropy　generation　is　also　established　for　variable　cross　section　of　the　microchannel.　The　simulation　encompasses　Reynolds　number　of　198-600,　relative　cavity　height　e　(1)/D　(h)　values　of　0-0.65,　relative　rib　height　e　(2)/D　(h)　range　of　0-0.2167.　The　results　show　that　the　effect　of　relative　rib　height　on　entropy　generation　is　significant,　while　the　relative　cavity　height　has　little　effect　on　it.　The　combined　effect　of　cavities　and　ribs　in　the　microchannel　has　better　performance　of　heat　transfer　than　the　smooth　microchannel　under　similar　conditions.　Extensive　simulations　are　conducted　to　collect　data　on　the　characteristics　of　heat　transfer　and　fluid　flow　in　a　micro　heat　sink　with　cavities　and　internal　ribs.　Using　these　data,　correlations　for　Nusselt　number　and　friction　factor　in　terms　of　Reynolds　number　and　the　geometry　of　cavity　and　rib　have　been　developed.