Ni-5B-xMo-3.5Cr-11.5V (mass%) model alloys with eight levels of Mo contents corresponding to Mo/B atomic ratio ranging from 0.8 to 1.5 were prepared to study the effects of the Mo/B atomic ratio on the mechanical properties and microstructures. The results indicated that transverse rupture strength (TRS) increased with increasing the Mo/B atomic ratio and showed the maximum value of 2.95 GPa at MoB=1.2 and then decreased with increasing the atomic ratio. Hardness increased monotonically from 82 HRA to 89.9 HRA with increasing the atomic ratio up to 1.3 and then gradually increased to 90.5 HRA with the atomic ratio up to 1.5. When the Mo/B ratio was 1.1 and 1.2, excellent TRS values were obtained because tetragonal M3B2 (M: Mo, B, Ni, Cr, V) complex boride particles were finely dispersed in the alloy. When the Mo/B ratio was less than 1.0, the TRS decreased due to the appearance of coarse orthorhombic M3B2 particles in addition to the tetragonal M3B2. Above the Mo/B ratio higher than 1.3, the TRS decreased due to the appearance of coarse tetragonal M5B3 particles.