In this work,we present a three dimensional (3D) micromixer which consists of two layers of spiral channels overlapped together in the vertical direction.Based on the principle of Dean effects,the 3D micromixer is designed by using a smooth channel twisted into double-layer spiral geometry with simple topological structure.This kind of structure is beneficial to produce,enhance and sustain the Dean vortices,which can perturb the laminar fluid effectively.In order to improve the mixing performance,the key parameters have been optimized by using the computational fluid dynamics software.The results indicate that the erect channel which is connected with the two layers of spiral channels plays a critical role for well mixing.Meanwhile,the effect of mixing has been identified in a fabricated glass-micromixer.The mixing efficiency of 90% has been achieved by optimizing the flow rate and the structure of the erect channel.Thus,the 3D micromixer has manifested high mixing efficiency and presents good practicability in the versatile microfluidic systems.