Effects of lamellar spacing on the mechanical properties of (Nb)/(Nb, Ti)₅Si₃ two-phase alloys with a fine lamellar structure have been investigated for an improvement in room-temperature toughness and high-temperature strength. Lamellar spacing of the alloy are controlled, without changing the volume fraction and composition of each phase, by varying annealing temperature for the decomposition of high-temperature phase (Nb, Ti)₃Si into (Nb) and (Nb, Ti)₅Si₃. A Time-Temperature-Transformation diagram (TTT diagram) was determined for a Nb-25 mol%Si-10 mol%Ti alloy. It was found that the decomposition of (Nb, Ti)₃Si in the Nb-Si-Ti system is kinetically faster than that of Nb₃Si in the binary Nb-Si system. Average lamellar spacing, λ , of the alloy is characterized by the degree of super-cooling, ΔT, following a relationship as “λ∝1⁄ΔT”. Coarse lamellar structure shows better room temperature compressive plastic strain before fracture and higher elevated temperature strength. The alloys investigated in this study show superior compressive strength at over 1000°C as compared with some commercial Ni-based superalloys.