Grain growth of quartz was investigated using two quartz samples (powder and novaculite) with water under pressure and temperature conditions of 1.0-2.5 GPa and 800-1100 • C. The compacted powder preserved a substantial porosity, which caused a slower grain growth than in the novaculite. We assumed a grain growth law of d n − d n 0 = k 0 f r H 2 O exp(−Q/RT)t with grain size d (µm) at time t (sec-onds), initial grain size d 0 (µm), growth exponent n, a constant k 0 (µm n MPa −r s −1), water fugacity f H 2 O (MPa) with the exponent r, activation energy Q (kJ mol −1), gas constant R, and temperature T in Kelvin. The parameters we obtained were n = 2.5 ± 0.4, k 0 = 10 −8.8±1.4 , r = 2.3 ± 0.3, and Q = 48 ± 34 for the powder and n = 2.9 ± 0.4, k 0 = 10 −5.8±2.0 , r = 1.9 ± 0.3, and Q = 60 ± 49 for the novaculite.