Pt-B/Al2O3 catalysts were prepared through chemical reduction of platinum ions with borohydride in a water/oil (W/O) microemulsion system comprising cetyltrimethyl ammonium bromide (CTAB), n-butanol, cyclohexane, and H2PtCl6 solution. Transmission electron microscope (TEM) and selected-area electron diffraction (SAED) analyses show that the active Pt particles on the catalysts are uniformly distributed as a Pt-B amorphous alloy. We studied the effects of microemulsion composition, reduction conditions, and preparation methods on the catalysts applied into m-chloronitrobenzene (m-CNB) hydrogenation. We also investigated the kinetics of m-CNB hydrogenation. The catalyst prepared via a single-microemulsion method exhibited much higher activity and even higher selectivity than that prepared with the aqueous solution method. Finally, we propose the mechanism for Pt-B effects on the catalytic performance of Pt-B/Al2O3 in m-CNB hydrogenation.