This　paper　investigates　fluid　flow　fictional　characteristics　in　microchannles　with　the　emphasis　of　entrance　effect　and　relative　roughness　influences.　The　experimental　results　indicate　that　there　is　no　earlier　transition　from　laminar　to　turbulent　flow,　and　the　critical　Reynolds　numbers　of　microchannels　with　the　relative　roughness　of　3.5%　and　the　hydraulic　diameter　of　345　μm　locate　at　the　range　of　2000~2300,　which　indicates　that　the　entrance　effect　does　not　influence　the　transition　from　laminar　to　turbulent　flow;　the　hydraulic　diameters　of　rectangular　copper　microchannels　with　relative　roughness　of　1.29%,　2.03%,　aspect　ratio　of　0.59,　0.63,　and　the　hydraulic　diameter　of　112.6　and　169.8　μm　are　investigated.　The　experimental　results　indicate　that　the　critical　Reynolds　number　locate　at　1600~1800.　The　experimental　results　indicate　that　micro　scale　effect　is　the　necessary　but　not　sufficient　condition　for　the　earlier　transition　from　laminar　to　turbulent　flow　which　is　caused　by　multiple　factors,　and　the　mechanism　of　the　micro　scale　fluid　flow　need　to　be　further　studied.