It is known that the reduction of grain size to nanometer scale is an attractive way to improve the mechanical properties of alloys. This reduction may be obtained by mechanical alloying. In this work, nanocrystalline powders with stoichiometrie Ni3Al composition were prepared by ball milling of AlNi or Al3Ni2 intermetallic compounds with addition of Ni powder. The powders at different stages of the milling process were investigated by X-ray diffraction and differential scanning calorimetry to study phase transformations that occurred during mechanical alloying and during subsequent heating in a calorimeter. It was found that in both cases at the first stage of the milling process, a metastable Ni(Al) solid solution was formed, and this phase remained as the only milling product. Upon heating of these powders in the calorimeter, the Ni(Al) solid solution became ordered and transformed into the Ni3Al intermetallic compound. The results obtained show that during heating in the calorimeter, a limited growth of grains occurred and nanocrystalline structure of powders was preserved.