Inspired by the microstructures on sharkskin, scientists found that the streamwise constant grooves over a surface can efficiently reduce the friction drag, but the mechanism beneath the phenomena remains an open question. In this paper, by using the direct numerical simulation, two types of riblets geometries (blade shape and triangular shape) that mimic the groove structures on the sharkskin were designed and simulated. It was found that the groove spacing has a close relationship with the drag variations. The drag reduction rate first increases with the increasing spacing before the critical value, and when it comes to the critical spacing, the drag reduction reaches the summit and starts to degrade, and finally the surface changes to a drag increase one. Both the critical spacing where the drag reduction peaks and the spacing when the groove surface starts to be drag-increased were determined for the blade-shaped and triangular-shaped grooves. For the mechanism of drag reduction below the critical spacing, it was found that the extra low-speed flow area near the grooved surface produces a smaller frictional drag. The drag increase in the case of larger spacing was shown to be related to the secondary flow around the grooves. The surface with large groove spacing can observe the strong secondary vortices which bring in extra frictional drag.