Thermal annealing effects on the magnetic properties of (Ga1-xMnx)As with various Mn mole fractions grown on GaAs (10 0) substrates by using molecular beam epitaxy were investigated. When thermal annealing was performed, the ferromagnetic transition temperature (T-C) of the (Ga1-xMnx)As thin films increased. The increase in the T-C originated from a decrease in the concentration of the Mn interstitial atoms or from an increase in the uncompensated Mn spin which contributed to the magnitude of the ferromagnetism. These results can help improve the understanding of the effect of thermal annealing on the magnetic properties of (Ga1-xMnx)As thin films grown GaAs substrates. (C) 2004 Elsevier B.V. All rights reserved.