Various ternary elements were added to observe the effects on the microstructural features of β+γ′ two-phase alloys. The microstructural features of β+γ′ two-phase (Ni(66)Al(34))(100-x)X(x)(X=Ti, Si, Nb) depended on the A(s) (austenite start) temperature of β′-martensite with alloying elements. For A(s)>250℃, a lamellar microstructure was found to form by the following phase transformation: Martensite→Ni5Al3→β+γ′. For As<250℃, two-type microstructures, mesh and Widmanstatten, were formed depending on the ternary element. When Ti or Nb was added as a ternary element, the β→Ni5Al3 transformation occurred very quickly. Conversely, this transformation proceeded very slowly in the case of Si addition, and the resultant microstructure assumed somewhat different features. Consequently, it could be suggested that the microstructures of NiAl/Ni3Al two-phase alloys are determined by not only the A(s) temperature but also by the β→Ni5Al3 transformation.