In this paper, 3D numerical simulations are performed to investigate the mixing process within an improved Tesla micromixer. This improved Tesla micromixer applies the flow separation/recombination and converging/diverging principles to enhance mixing. A portion of the working fluid, which separates from the main flow, enters the Tesla side branch and mixes with the main flow again at the exit of the Tesla unit. The tested volume flow rate ranges from 1 μL/min to 100 μL/min. Grid independence is carried out to minimize the effect of numerical diffusion. Optimization is done to determine three parameters, which are the gap ratio (H/W), the mixing cell number (N), and the angle at the gap inlet (β). The effects of these three parameters on mixing are investigated at a volume flow rate of 100 μL/min. The simulation results show that the gap ratio is the most important factor. Three parameters are selected as H/W = 50/200, N = 10 and β = 90° for further investigation. The traditional Tesla micromixer is also simulated for comparison with the present design. The mixing efficiency is approximately 60% in the range of the tested volume flow rate. The improved micromixer has better mixing efficiency than the traditional Tesla micromixer when the volume flow rate is less than 50 μL/min.