Experiments of vacuum annealing were carried out for nanoparticles of TiAl intermetallic compound prepared by the flow-levitation method. And the effect of annealing parameters on the phase transformation of TiAl alloy was investigated by X-ray diffraction analysis. The results show that more g-TiAl phase is transformed from a2-Ti3Al phase with the increasing of annealing temperature and time. And the grain size of two phases increases accordingly. This phase transformation is promoted after nanoparticles are compressed into a bulk by vacuum-pressing. The nanocrystalline alloy with pure g-TiAl phase is obtained after the bulk compressed from mixed phase nanoparticles is vacuum-annealed.