The turbulent flow of surfactant solution in the wide-rib rectangular grooved channels was studied by direct numerical simulation. Moreover, the variations of near-wall streamwise vortices with time were discussed and the distributions of streamwise vortex radius, swirling strength and density were quantitatively investigated. It was found that the influence of microgrooves on the fluid mainly occurred within the buffer layer and microgrooves could induce numerous streamwise vortices with small size and swirling strength within the grooved valleys. The drag-reducing enhancement mechanism of microgroove in the surfactant solution could be mainly considered as the competing results between the restriction effect and tip effect of microgroove, and the essential factor should be the numerous secondary streamwise vortices with small size and swirling strength within the grooved valleys. Furthermore, a predicted method for the optimal drag-reducing size of microgroove was proposed, and the prediction values agreed well with the numerical results. (c) 2018 American Institute of Chemical Engineers AIChE J, 64: 2898-2912, 2018