Mean grain sizes of σ phase particles in 25%Cr-20%Ni austenitic stainless steel have been measured during aging at temperatures between 1123 and 1223 K in order to make theoretical analyses of the growth kinetics of the grains. It has been found that both of the γ and σ phases follow the fourth powered law as described below:(This article is not displayable. Please see full text pdf.) \ oindentwhere R, R0, K and t are, respectively, a mean radius of the grains at time, t, an initial radius at time=0, a growth constant and an aging time. The growth constant is, however, different between the growing phases, γ and σ, as follows:(This article is not displayable. Please see full text pdf.) \ oindentwhere K′s are, respectively, the growth constants for superscripted phases, and β is the Zener parameter and f the volume fraction of the σ phase. Furthermore, the growth of the σ phase has been found to be controlled by the diffusion along the grain boundaries between precipitates and matrix, i.e., by the mechanisms of Ostwald ripening. The growth of the γ phase has taken place under the influence of pinning action by the precipitated σ particles: The driving force to grow grains of the γ phase has been supposed to be approximately equal to one-sixth of that to grow the σ particles.