A study on the kinetics of grain growth of an Mg-12.1 wt%Cu alloy produced by mechanical alloying was carried out. The grain sizes of as-mechanically alloyed powder and of cold-compacted annealed powder were determined from the broadening of X-ray lines. The grain size deceases initially due to recrystallization and then increases gradually, and finally ceases to reach an ultimate value regardless of annealing time. From isothermal anneals, the grain growth kinetics can be described by D-n - D-0(n) = ct, where n (n = 5 to 8) is a constant essentially dependent on the annealing temperature. The activation energy for grain growth Q has been determined to be 118 kJ/mol, which is longer by 26 kJ/mol than that for pure magnesium. Second-phase intermetallic particle Mg2Cu produced during ball-milling influences not only on activation energy but also on exponent of the kinetic equation.