Various investigations have been attempted to improve the low temperature ductility of Nb-silicides by microstructural control while they show superior high temperature strength. Present authors have focused on the microstructure evolution through the eutectic and eutectoid reactions in Nb-rich portion of Nb-Si binary system, and with small amounts of additives (Zr or Mg) alloys large Nb grains with fine suicide (α-Nb5Si3) particles have been obtained, which is attractive for high temperature use. For further understanding of this phenomenon, the present study has two objectives; one is to apply the advanced solidification technique for further microstructure control, and the other is to investigate the effect of co-existence of Zr and trace amount of Mg on the microstructure evolution during the eutectoid reaction in terms of the interfacial energy between phases. EBSD analysis revealed that uni-directionally solidified alloy show the same crystallographic orientation relationship (O.R.) between Nb and α-Nb5Si3 with that in arc-melted alloy having the same composition. On the other hand, Mg-doped alloy containing Zr shows an O.R. which was not observed in previous works. This implies that Mg doping is effective to control the interfacial energy between Nb and α-Nb5Si3 even in Nb-Si-Zr alloys. Two-step heat-treatment is found to be effective to obtain finer microstructure, and a further investigation on the controlling factors of eutectoid decomposition will provide a proper route to well-controlled microstructures.