Drag　reduction　effect　of　surfactant　solution　could　be　enhanced　in　longitudinal　microgroove　channel,　but　their　collaborative　drag-reducing　mechanism　was　unclear.　In　order　to　provide　the　basis　for　the　study　on　their　coupling　drag-reducing　mechanism,　the　investigation　on　the　drag-reducing　mechanism　of　longitudinal　microgroove　in　water　was　necessary.　The　purpose　of　this　study　was　to　investigate　the　drag-reducing　mechanism　of　wide-rib　rectangular　microgroove　with　a　thickness-to-spacing　ratio　t/s　of　0.25　in　water　by　direct　numerical　simulation　and　transient　analysis　of　vortex.　It　was　found　that　the　drag-reducing　mechanism　of　longitudinal　microgroove　in　water　could　be　considered　as　the　results　competed　between　the　“tip　effect”　and　the　“restriction　effect”　of　microgroove.　The　former　could　cause　a　drag-　increasing　effect,　but　the　latter　might　cause　a　drag-reducing　effect.　There　is　a　higher　frictional　stress　on　the　grooved　tip　caused　by　the　“tip　effect”　of　grooves.　The　bottom　of　grooved　valley　shows　much　smaller　wall　shear　stress　due　to　the　“restriction　effect”　of　grooved　valleys.　Microgrooves　present　net　drag　reduction　when　the　latter　effect　is　larger　than　the　former.　Moreover,　the　numerous　secondary　streamwise　vortices　with　smaller　scale,　which　were　induced　by　the　grooves　and　stayed　within　the　grooved　valleys　and　then　prevented　the　small　near-wall　vortices　from　intruding　into　the　grooved　valleys,　should　be　the　essential　factor　and　the　prerequisite　of　drag　reduction　of　microgrooves.　Copyright　©　2018-2019　by　JSME