The phase diagram of the Ni-Mo-B ternary system in the region of less than 50 mol%B was constructed by thermodynamic calculation. We found three ternary eutectic points and three ternary peritecto-eutectic points as follows:E 1 : L(1365 K, 71.5 mol%Ni-6.0 mol%Mo-22.5 mol%B)=(Ni)+Ni 3 B+NiMo 2 B 2 .E 2 : L(1355 K, 62.5 mol%Ni-2.5 mol%Mo-30.5 mol%B)=Ni 3 B+Ni 2 B+NiMo 2 B 2 .E 3 : L(1445 K, 42.0 mol%Ni-30.6 mol%Mo-10.3 mol%B)=(Ni)+NiMo+NiMo 2 B 2 .P 1 : L(1812 K, 34.9 mol%Ni-42.3 mol%Mo-22.8 mol%B)+MoB=Mo 2 B+NiMo 2 B 2 .P 2 : L(1633 K, 42.3 mol%Ni-40.4 mol%Mo-17.3 mol%B)+Mo=Mo 2 B+NiMo 2 B 2 .P 3 : L(1478 K, 53.5 mol%Ni-33.7 mol%Mo-12.8 mol%B)+Mo=NiMo+NiMo 2 B 2 .The calculated phase diagram is expected to be useful for the development of new Ni-based heat-, corrosion- or wear-resistance alloys.