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.