In this paper, the shape instabilities of Nb in in situ Cu-Nb microcomposite wires after exposed to different annealing treatments have been analyzed using scanning electron microscopy and transmission electron microscopy technologies. The results suggest that the thermal stability is related to misorientation among the adjacent grains at the triple joint. Most of the triple joints are composed of low-angle grain boundaries in Nb ribbons and Cu-Nb interfaces of (111)(Cu)//(011)(Nb). These triple joints provide dragging force to interface motion so that neither the grains nor the interface boundaries show substantial changes below 500 degrees C. Above 500 degrees C, the Nb ribbons start to dissociate at the triple joints within Nb phase due to the stored energy by misorientation/distortion in Nb. Grooves and pits formed at these regions tend to promote the spheroidization of the Nb phase. Such results may enrich the studies on the microstructure evolution of Cu-based microcomposite.