The　characteristics　of　liquid　flow　in　the　microchannels　with　circular　and　trapezoidal　cross-sections　were　studied　by　numerical　simulation.　The　friction　factor　of　liquid　flow　in　microchannels　was　investigated　with　different　pressures,　the　sizes　and　roughness　of　microchannels.　The　different　flow　regimes　of　laminar　and　turbulence　were　also　analyzed　in　the　microtubes.　In　addition,　the　range　of　a　transition　Reynolds　number　can　be　inferred　from　the　friction　factor　profiles　under　different　Reynolds　numbers.　The　friction　factors　in　the　microchannels　decrease　with　the　increasing　of　Reynolds　numbers　gradually.　The　Reynolds　number　at　transition　from　laminar　to　turbulent　flow　changes　with　the　cross-section＇s　equivalent　diameter.　At　the　same　Reynolds　number,　the　friction　factor　increases　with　the　roughness　of　the　inner　wall　of　the　microchannel　when　the　flow　is　turbulent.