Creep experiments were performed on synthetic fine-grained garnetite to investigate the flow strength of the Earth's subducting slabs. Experiments were conducted at temperatures (T) of 1373--1543 K and total pressure (P) of 0.1 MPa in controlled atmospheres of fO2=10-17--10-8 MPa. The mechanical data indicate a grain-size sensitive diffusion flow and the creep behavior can be described by an equation of the form: ϵ˙=(5.32¿3.10)¿10-6/Td2.5¿0.3 fO20&sgr;1.1¿0.2 exp(-347¿46kJ/mol/RT) where T in Kelvin, d in meter, &sgr; and fO2 in MPa. Based on the diffusivities (D) calculated from creep and diffusion experiments, we proposed that grain boundary diffusion is the dominant mechanism for high temperature creep of the fine-grained garnetite. Normalized creep strength of the garnetite is found to be comparable to those of feldspar and olivine in diffusion creep regime, suggesting that garnetite may not form a strong layer in the subducted oceanic lithosphere if it deforms by grain boundary diffusion creep. ¿ 2000 American Geophysical Union