This Ph.D thesis investigates the mixing performances in a micro T-mixer and a cross-shaped micromixer incorporated with static mixing elements. The micromixers are fabricated on a silicon substrate sandwiched between a glass cover plate and a supporting polystyrene plate. Design calculations and computer simulations are performed to predict the flow properties and mixing performances in the micromixers. The mixing performance is characterized by mixing a commercial blue dye and de-ionized water. The results are further verified by mixing two chemical solutions that produce a distinct colour change when fully mixed. Complete mixing is observed in the micro T-mixers when the Reynolds number in the mixing channel is between 400 and 500. The corresponding times are in the range of one millisecond. Complete mixing is not observed in the cross-shaped micromixers investigated but significant improvement in mixing performance is nevertheless achieved. The rapid mixing in micro T-mixers is largely explained by the asymmetrical flow conditions at the inlets that result in swirling flow in the mixing channel that significantly enhances the mixing performance. The short mixing time achieved by the micro T-mixers makes them suitable for the analysis of fast reactions involving expensive samples.