Drag-reducing　surfactant　solution　can　provide　a　large-eddy　environment　for　longitudinal　microgrooves　and　may　realize　the　complementarity　between　their　drag-reduction　mechanisms.　In　this　work,　the　collaborative　drag　reduction　performance　of　surfactant　solution　and　longitudinal　microgrooves　was　experimentally　studied　to　verify　the　speculation　about　their　complementary　possibility.　The　mixture　aqueous　solution　of　cationic　surfactant　(cetyltrimethyl　ammonium　chloride)　and　counterion　salt　(NaSal)　was　tested　in　the　smooth　and　two　longitudinal　microgroove　channels　respectively　at　the　mass　concentrations　of　0.16-0.47　mmol/L.　It　was　found　that　the　drag　reduction　performance　of　surfactant　solution　was　enhanced　by　the　longitudinal　microgrooves.　The　drag-reduction　mechanisms　of　microgrooves　in　water　and　surfactant　solution　were　illustrated　by　the　competition　between　the　＂peak　effect＂　and　the　＂restriction　effect＂　of　microgroove.　Moreover,　the　＂second　peak　effect＂　was　proposed　to　explain　the　drag　reduction　enhancement　mechanisms　for　surfactant　flow　in　microgroove　channels.　The　groove　with　a　larger　size　and　roughness　which　might　increase　the　drag　in　water　could　still　enhance　the　drag　reduction　effectiveness　of　surfactant　flow,　and　had　a　lower　critical　temperature　and　critical　Reynolds　number　in　surfactant　solution,　indicating　a　promising　application　in　the　heat　transfer　and　drag　reduction　field.　Moreover,　the　results　of　particle　image　velocimetry　of　smooth　channel　indirectly　verified　that　the　drag　reducing　mechanism　of　microgroove　was　related　to　the　turbulent　vortex　scale　and　the　restriction　effect　on　near-wall　vortices.　(C)　2016　Elsevier　Ltd.　All　rights　reserved.