NiCoB alloy catalyst has been reported to be a good catalyst for the hydrogenation of p-chloronitrobenzene to p-chloroaniline. The objective of this study was to investigate the effects of Mo content on the catalytic properties of NiCoB in the hydrogenation of p-chloronitrobenzene. A series of Mo-doped NiCoB nanoalloy catalysts with various Mo contents were prepared by chemically reacting nickel acetate, cobalt acetate, ammonium heptamolybdate, and sodium borohydride in a 50 vol % water/methanol solution at 298 K under and N-2 gas curtain with vigorous stirring. The Ni/Co atomic ratio was fixed at 10. An excess amount of NaBH4 was used [B/(Ni + Co) atomic ratio = 3] to fully reduced Ni and Co cations. The catalysts were characterized with nitrogen sorption, X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. The catalysts were tested for liquid-phase hydrogenation of p-chloronitrobenzene at 353 K and 1.2 MPa H-2 pressure. Even adding a small amount of Mo had significant effect on activity and selectivity of NiCoB. The activity increased with an increase of Mo content until a Mo/Ni atomic ratio of 0.6 and then decreased. The catalyst with the atomic ratio of Mo/Ni = 0.6 had the highest surface area and the highest activity and selectivity of p-chloroaniline. The particle size of Mo-NiCoB decreased with an increase of Mo content. The crystallization temperature extended upward with the increase in Mo content, indicating that the molybdenum species suppressed the growth of the crystalline structure of NiCoB and helped the NiCoB catalyst maintain its amorphous state. Molybdenum was mainly in the form of hydroxide and acted as a spacer to prevent NiCoB from aggregation/agglomeration. Adding the proper amount of molybdenum in NiCoB catalysts increased the surface area, activity, and selectivity. An overdose of molybdenum had an opposite effect on the NiCoB catalyst. The optimum atomic ratio of Mo/Ni was 0.6.