Due to their high yield, microreactors can be considered for biodiesel production. Efficient premixing of reactants is crucial to get high reaction rates. An efficient uniflow passive micromixer based on multi-lamination principle is developed in this paper. Proposed design combines high mixing quality with significantly lower pressure drop compared with other recent publications. Optimization is performed numerically for low Reynolds numbers, which are usually encountered for biodiesel due to mixture high viscosity and small size. Design optimization is made in different steps, starting from a simple but efficient 'preliminary' design. Different enhancements are introduced and optimized one after the other, to reduce the total number of free parameters at each step. Final micromixer design is studied for different fluid mixtures. Comparison of results with other recent publications gave quite satisfactory results.