In order to spheroidize -Nb5Si3 strengthening phase embedded in Nb matrix for attaining a good room temperature toughness of Nb-Si alloy, the authors have proposed a microstructure control technique by combining eutectic and eutectoid reactions. Nb3Si intermetallic compound formed during solidification is a key phase for the microstructure control, but its stability is very sensitive to the alloying elements. Nb3Si disappears by adding as small as 3 at% of W and Mo, while these elements are very effective for the solid solution strengthening of Nb phase. For a further alloy development, establishment of an alloy design concept based on the control of phase stability of Nb3Si is needed. Similarly to ferrous alloys such as stainless steels where Cr and Ni are added to control the stability of bcc phase and fcc phase, two alloying elements (one is a stabilizing element and the other is a destabilizing element for Nb3Si phase) are added to a Nb-Si binary master alloy and their microstructure is investigated using SEM. The stabilizing element Ta is found to enlarge the composition area where Nb3Si exists even with the destabilizing element Mo, and it is confirmed that the phase stability concept is useful for designing Nb-Si based alloys.