In this study, direct numerical simulations are conducted to exhibit the viscous drag reduction effect of equilateral triangle riblets at Mach 0.85 and investigate the optimal groove width for maximum drag reduction. Additionally, the Mach number effect on their drag reduction performance is elucidated by comparing the drag reduction effect of riblets at Mach 0.85 with that at Mach 0.2. The computational results indicate that largest reduction in drag, approximately 11%, is achieved at Mach 0.85