The　phenomenon　that　flow　resistances　are　higher　in　micro　scale　flow　than　in　normal　flow　is　clarified　through　the　liquid　viscosity.　Based　on　the　experimental　results　of　deionized　water　flow　in　fused　silica　microtubes　with　the　inner　radii　of　2.5　mu　m,　5　mu　m,　7.5　mu　m,　and　10　mu　m,　respectively,　the　relationship　between　water　flow　velocity　and　pressure　gradient　along　the　axis　of　tube　is　analyzed,　which　gradually　becomes　nonlinear　as　the　radius　of　the　microtube　decreases.　From　the　correlation,　a　viscosity　model　of　water　flow　derived　from　the　radius　of　microtube　and　the　pressure　gradient　is　proposed.　The　flow　results　modified　by　the　viscosity　model　are　in　accordance　with　those　of　experiments,　which　are　verified　by　numerical　simulation　software　and　the　Hagen-Poiseuille　equation.　The　experimental　water　flow　velocity　in　a　fused　silica　microtube　with　diameter　of　5.03　mu　m,　which　has　not　been　used　in　the　fitting　and　derivation　of　the　viscosity　model,　is　proved　to　be　comsistent　with　the　viscosity　model,　showing　a　rather　good　match　with　a　relative　difference　of　5.56%.