Mixing characteristics inside a microfluidic liquid slug using the Computational Fluid Dynamics (CFD) simulations are reported. Slug-based microfluidics offers rapid mixing by internal circulation and transport with narrow residence time distribution, making it suitable for precise reaction and mixing operations. The simulation results show that the circulation flow occurs in the slug and stagnant regions exist in the front and tail parts and in the center of recirculation vortices. This circulation flow greatly reduces mixing time as compared with the mixing time when mixing is driven by only molecular diffusion. The results also provide insights into the influences of operating parameters on slug-based mixing rates. Based on the simulation results, the modified Peclet number, Pe∗ = Usds2/lD, is proposed for designing mixing in liquid slugs. A novel method using Pe∗ to estimate mixing rates and design liquid slugs to obtain desired mixing rates is discussed and confirmed experimentally. In conclusion, design of microfluidic liquid slug for measuring kinetics of fast reactions and mixing operations is made possible by realizing instant mixing and narrowed residence time distribution.