A novel passive micromixer based on multiple baffles and a submergence scheme was designed, and its mixing performance was simulated over a wide range of Reynolds numbers ranging from 0.1 to 80. The degree of mixing (DOM) at the outlet and the pressure drop between the inlets and outlet were used to assess the mixing performance of the present micromixer. The mixing performance of the present micromixer showed a significant enhancement over a wide range of Reynolds numbers (0.1 ≤Re≤ 80). TheDOMwas further enhanced by using a specific submergence scheme. At low Reynolds numbers (Re< 5), submergence scheme Sub24 produced the highestDOM, approximately 0.57, which was 1.38 times higher than the case with no submergence. This enhancement was due to the fluid flowing from or toward the submerged space, creating strong upward or downward flow at the cross-section. At high Reynolds numbers (Re> 10), theDOMof Sub1234 became the highest, reaching approximately 0.93 forRe= 20, which was 2.75 times higher than the case with no submergence. This enhancement was caused by a large vortex formed across the whole cross-section, causing vigorous mixing between the two fluids. The large vortex dragged the interface between the two fluids along the vortex perimeter, elongating the interface. The amount of submergence was optimized in terms ofDOM, and it was independent of the number of mixing units. The optimum submergence values were 90 μm for Sub24 andRe= 1, 100 μm for Sub234 andRe= 5, and 70 μm for Sub1234 andRe= 20.