Thermal behaviours of mechanical alloyed Ti50Al50 powders in a nitrogen atmosphere are investigated in this paper. X-ray diffraction and differential thermal analysis were used to determine their characteristics. At the initial milling stage, large amounts of defects were introduced and the grain sizes were gradually refined. The enthalpy changes of formation of gamma-TiAl and alpha(2)-Ti3Al were decreased with increasing milling times. No obvious dissolution of nitrogen into the powder particles occurred at this stage. With increasing milling time, an amorphous phase containing nitrogen gradually occurred. The amorphous phase and small amounts of Ti solid solution were obtained after milling for 30 h in a N-2 atmosphere. The thermal process included two stages. Firstly, the amorphous phase crystallized at low temperature and resulted in the formation of a nanophase; secondly, the grain growth of this nanocrystalline phase occurred at high temperature. The annealing products are different for the milling products obtained at the initial stage (gamma-TiAl + alpha(2)-Ti3Al) and final stage (gamma-TiAl + Ti2AlN), which is attributed to the different nitrogen contents in the milled products. The activation energies for the crystallization and grain growth are 251.9 and 296.9 kJ mol(-1), respectively.