The　microfluid-induced　nonlinear　free　vibration　of　microtubes　is　studied　in　this　paper.　A　derivation　of　the　nonlinear　equation　of　motion　is　presented　based　on　Hamilton＇s　principle　and　a　modified　couple　stress　theory.　The　geometric　nonlinearity,　arising　from　the　midplane　stretching,　is　taken　into　account.　The　modified　couple　stress　theory　is　used　to　capture　the　micro-structure　dependent　size　effects　when　the　microtubes　are　at　micron-　and　submicron　scales.　The　static　postbuckling　problem　is　then　studied　and　the　size-dependent　postbuckling　configurations　are　analyzed.　The　approximate　solution　to　the　nonlinear　free　vibration　is　obtained　using　the　homotopy　analysis　method.　The　influences　of　internal　material　length　scale　parameter,　outer　diameter,　flow　velocity,　and　Poisson＇s　ratio　on　the　dynamic　behavior　are　discussed　in　detail.　(C)　2013　Elsevier　Ltd.　All　rights　reserved.