Smoothed Particle Hydrodynamics, SPH, is a pure Lagrangian method. SPH has several advantages over other fluid dynamical methods which may encounter difficulty with deformable boundaries and multiphase fluid flows. SPH method is used for simulation of two fluids flow in an active micromixer and for this case, continuity, momentum and mass diffusion equations are solved. Several boundary conditions on the solid wall and Inlet/Outlet have been investigated. Three types of boundary particles, virtual particles, mirror ghost particles and Morris dummy particles are introduced. Virtual particles are useful for modeling nonviscous fluid flow and easily satisfy no penetration boundary condition on solid surface. Mirror ghost particles are useful for modeling no slip boundary condition on solid surface and Morris dummy particles are used for modeling curved solid surfaces. The Couette flow in a periodic channel is modeled and also the Poiseuille between two parallel plates with periodic and nonperiodic channel is simulated. In these cases the SPH velocity profiles have close agreement with the series solution transient velocity profile. Flow field and mixing process in a closed chamber with 9 microstirrers is modeled. Three type of microstirrer rotation is investigated and the concentration contours during the mixing process is presented. Finally the mixing process in horizontal and Y shape micromixer is simulated and the concentration contours in the channel are presented. The effect of several design parameters such as Re, microstirrer length, angular velocity and oscillation angle are investigated and at the end the efficient case with the best mixing efficiency is presented. Key Words SPH, Particle method, Micromixer, Microstirrer