The hydration of two japonica(Akibare and Milyang 15) and four indica(Milyang 30, Suweon 287, Suweon 294 and Iri 342) rice varieties was investigated in terms of mathematical rate equation. The hydration rate at temperatures of 4∼32℃ was examined by a weighing method. The absorption of water was directly proportiporal to the square root of the hydration time(t) and was described by the diffusion equation: 1-M￣=(2/√ π￣)(S/V) √Dt, where M￣ is dimensionless moisture ratio, S/V is the surface-to-volume ratio and D is diffusion coefficient. The average D value was given by the Arrhenius relation: D=D_o exp(-E_o/TR). The activation energy was 4∼5㎉/mole. The rice samples could be classified into three groups based on hydration kinetics: Milyang 30, Suweon 287; Akibare-Milyang 15; and Suweon 294-Iri 342.