This paper reports on a lattice Boltzmann simulation of laminar mixing in a coaxial microreactor at low Reynolds numbers. The main focus of the study was to compare the effectiveness of a multi-holed baffle plate, a set of $2 \,{\times}\, 2$ square bars, and a flat-plate arrangement on the enhancement of reagent mixing inside the reactor. It was found that all three mixers increased mixing through the mechanism of generating coherent structures, which in turn increased interface contact between the fluid streams. However, for this particular microreactor, the efficiency of each mixer depended on its ability to generate coherent structures with high helicity and thereby inducing a swirl motion to the flow.