Reaction boronizing sintering technique has successfully brought a development of Mo₂NiB₂ complex boride base cermets. The Mo₂NiB₂ boride base cermet consists of M₃B₂ type complex boride as a hard phase and a Ni base binder. Accrding to the previous investigation, it had been clarified that the additions of Cr and V to the cermet changed the boride phase structure from orthorhombic to tetragonal and resulted in a remarkable improvement of mechanical properties such as transverses rupture strength (TRS) and hardness associated with microstructural refinement. In this investigation, the effects of Mn on the mechanical properties and microstructure of the cermets were studied by using model cermets with composition of Ni-5mass%B-58.6 mass%Mo and Ni-4.5mass%B-47.9 mass%Mo-12.5 mass% V with five levels of Mn content from 0 to 10mass%. Appearance of significant microstructural refinement by an addition of Mn, especially suppression of grain growth of the boride resulted in the improvement of strength and hardness. The maxmum TRS obtained at 2.5mass%Mn of V added cermet were 3.5 GPa with hardness of 87 HRA.