The undercooling and solidified structure of Fe-Nb binary alloys and Fe-0.1mass%C-Nb ternary alloys were investigated by changing Nb concentration from 0.5 to 6mass%. A glass-encased method was used for achieving high undercooling, in addition to a conventional solidification method. The specimen was furnace-cooled followed by quenching from 1723K or air-cooling from 1473K. As a result, the degree of undercooling in the Fe-Nb alloys solidified by the glass-encased method was higher than that in the specimen solidified by the conventional method, while that in the Fe-0.1mass%C-Nb alloy solidified by the glass-encased method was not so high. The solidified structure in all alloys that were quenched at gamma formation temperature has dendritic morphology in spite of delta/gamma transformation. The secondary arm spacing of the dendrite became fine with increase in niobium concentration.