Nb-Ti-Si base ultrahigh temperature alloys that possess higher melting points, relatively lower densities and attractive high temperature strength have received worldwide attention for their potential applications as next-generation turbine blade materials. In this work, integrally directional solidification of an Nb-Ti-Si base ultrahigh temperature alloy was conducted at different withdrawing rates (2.5, 5, 10, 20, 50 and 100 μm/s) with a constant melt temperature of 2000℃. Effect of solidifying rate on the integrally directionally solidified eutectic microstructure and solid/liquid interface morphology of this alloy has been investigated by XRD, SEM and EDS, and its directional solidification behavior has been discussed. The results show that the directionally solidified microstructure is mainly composed of petal-like Nb_ss/α(Nb,X)₅Si₃ eutectic colonies (Eutectic I) and coupled grown lamellar Nb_ss/γ(Nb,X)₅Si₃ eutectic (Eutectic II) which distributed in the intercellular area. The Eutectic I and Eutectic II are both aligned straight and uprightly along the growth direction. When the solidifying rate increases from 2.5 μm/s to 100 μm/s, the microstructure becomes finer and finer, and the petal-like eutectic colonies evolve from round morphology to tetragonal morphology. Either silicides or fine eutectics locate in the centers of round eutectic cells, while cross-like Nb_ss locates in the centers of tetragonal eutectic cells. Eutectic II exhibits a well-aligned lamellar structure on longitudinal-section. The solid/liquid interface of the alloy undergoes an evolution from cellular dendrite, dendrite and finally to cellular dendrite morphologies.