The effects of Si addition on the microstructures and mechanical properties of dual two-phase Ni₃Al and Ni₃V intermetallic alloys with a composition of Ni₇₅Al₉V₁₃Nb₃ (expressed by at%) were investigated, focusing on the substitution manner of Si for Ni, Al or V. Solubility limit of Si in the present microstructure was less than 2 at% Si irrespective of the substitution manner of Si for Ni, Al or V. The eutectoid microstructure in the channel region was degenerated when Si was substituted for V. Third-phase dispersions containing Nb, i.e., Ni₁₆Si₇Nb₆ (G phase: D8a) were present at grain boundaries of the alloys exceeding the solubility limit. In the alloy in which Si was substituted for Ni, unit cell volume of the constituent Ni₃Al phase increased while that of the constituent Ni₃V phase little changed. In the alloy in which Si was substituted for Al, unit cell volumes of both phases decreased. The alloys in which Si was substituted for Al or Ni hardened while the alloy in which Si was substituted for V softened. The hardening in the former alloys was attributed to solid solution hardening due to Si substituted for solvent atoms Ni, Al and/or V while the softening in the latter alloy was attributed to the degenerated eutectoid microstructure in the channel region. Also, it was shown that the third-phase dispersions little affected hardness as well as yield strength.