The steady-state grain size of Earth materials undergoing solid state flow is estimated based on a nucleation-and-growth model of dynamic recrystallization. Assuming a nucleation mechanism of subgrain rotation, the mean diameter d of recrystallized grains is obtained as d/b=A(&sgr;/&mgr;)-p exp[-((Qgb-Qv)/mkT)>, where b is the length of the Burgers vector, &sgr; is differential stress, &mgr; is the shear modulus, Qgb is the activation energy for the jump of an atom across the grain boundary, Qv is that for self-diffusion in the grain volume, k is the Boltzmann constant, T is temperature, A is a constant, p=1.25 and m=4 for intracrystalline nucleation, and p=1.33 and m=3 for grain-boundary nucleation. The exponent p=1.25~1.33 agrees well with available data for high-temperature dislocation creep of rock-forming minerals. A weak negative dependence of grain size on temperature is expected from this theory. ¿ 1998 American Geophysical Union