The objective of this study was to investigate the effect of Mo content on the catalytic properties of Mo-NiB nanoclusters in the hydrogenation of p-chloronitrobenzene. A series of Mo-doped NiB nanoalloy catalysts with various Mo contents were prepared by chemical reduction method. An excess amount of NaBH4 was used (B/Ni atomic ratio = 3) to fully reduce Ni. Even adding a small amount of Mo on NiB had a significant effect on the activity and selectivity to p-chloroaniline. The catalyst with the atomic ratio of Mo/Ni = 0.4 had the highest activity and selectivity to p-chloroaniline. The particle size of Mo-NiB decreased with an increase of Mo content, indicating that the molybdenum species suppressed the growth of the crystalline structure of NiB and helped the NiB nanocluster maintain its amorphous state. Mo-NiB formed a nanoalloy, and no discrete phase was found. High-resolution transmission electron microscopy (HRTEM) images confirmed that the shape of Mo-NiB was spherical and the particle size was in the range of 3-5 nm. The effect of molybdenum on the catalytic performance of the amorphous NiB catalysts was 2-fold. Molybdenum not only acted as a spacer but also donated partial electron to Ni. Since the -NO2 was more electronegative than -Cl, -NO 2 was supposed to occupy the active site on the Ni catalytic surface at the start of the reaction. -NO2 adsorbed on the catalyst surface is hydrogenated to form p-ceric ammonium nitrate (p-CAN), which was further desorbed. Therefore, the selectivity of p-CAN would increase as the Mo contents increased. It should be noticed the overdose of Mo would cover the active sites and decreased the activity. In addition, oxygen was even more electronegative, and alloying B could engage the oxygen to activate the polar -NO2 group of p-CNB. The -NH2 of p-CAN might adsorb on the surface, alloying B and coordinating with each other. Hence, it would improve the selectivity of p-CAN by depressing the dehalogenation reaction.