Micro mixers are an integral part of several micro fluidic devices like micro reactors or analytical equipment. Due to the small dimensions, laminar flow is expected a priori in those devices while the mass transfer is supposed to be dominated by diffusion. A detailed numerical CFD-study by CFDRC-ACE+ of simple static mixers shows a significant deviation from strictly laminar flow in a wide range of Reynolds numbers Re, channel dimensions, and types of cross sections (square, rectangular, trapezoidal). With increasing flow velocity and Re number the flow starts to form vortexes at the entrance of the mixing channel. The vortexes are symmetrical to the symmetry planes of the mixing channel, both for the rectangular and the trapezoidal cross sections investigated here. With further increasing velocity the flow tends to instabilities, which causes a breakup of the flow symmetry. These instabilities are generally found in T-shape mixers with symmetrical flow conditions, but not always in Y-shape mixers or with asymmetrical flow conditions. Within the laminar flow regime diffusive mass transfer is dominant. In this case the mixing quality at constant channel length becomes worse with increasing velocity. This effect can almost be equalized by the onset of the vortex regime, which enhances the mass transfer by convective transport. This paper shows the mixing quality at a certain length for different geometrical parameters and flow conditions.