Three kinds of Mo-Si-B ternary alloys and a 1 at.% Al added Mo-Si-B alloy with the compositions near Mo-8.7 at.% Si-17.4 at.% B that is in the Mo5SiB2 and Mo two-phase compositional region were produced by Ar arc-melting followed by the heat treatment at 1800 °C for 24 h. These alloys have the characteristic fine microstructure composed of small Mo solid solution (Moss) particles in the Mo5SiB2 (T2) matrix with the primary phase (Moss or T2 depending on composition). The volume fraction of the Moss particles ranges from 25.5 to 30.5 % and its average size from 3.0 to 6.4 μm in the fine microstructure of the alloys. Micro cracks were introduced by Vickers hardness tests into the microstructures, and their propagation is disturbed by the small Moss particles. Thus, each hardness value seems to relate to the cracking behavior around each indent. On the other hand, Vickers hardness values do not show correlation with the volume fraction of the Moss particles, but clearly decrease with increasing the average particle size of Moss. Therefore, it should be concluded that the increase in the particle size of Moss could enhance the toughness of the Mo5SiB2/Mo-based alloys effectively by ductile phase toughening.