To investigate the phase transformation behaviors of mechanically alloyed Nb-25 at%Al powders, the mixed Nb-25 at%Al powders were mechanically alloyed in SPEX 8000 Mixer/Mill. Mechanical alloying(MA) time was varied between 0.5 hour and 72 hours. The phase formation behaviors of these mechanically alloyed powders were examined using X-ray diffraction(XRD), scanning electron microscopy(SEM), transmission electron microscopy(TEM), and differential thermal analysis(DTA). Appreciable amorphization started from 6 hours of MA. The powders mechanically alloyed for 10 hours were in almost amorphous phase. DTA results showed that the powders mechanically alloyed for 12 hours had a strong exothermic peak about$600^{circ}C$, whereas the powders mechanically alloyed for 6 hours had two exothermic peaks. The first peak was found to be due to the stress relief effect and the second one due to the formation of$Nb_{3}Al,Nb_{2}Al and Nb_{2}C$phases by crystallization.