© 2014. American Geophysical Union. All Rights Reserved. We deformed two natural magnesite aggregates (grain sizes of 1 versus 100 μm) over a wide range of temperatures (400-1000°C) and strain rates (10-7-10-4/s) in order to determine the deformation mechanisms of magnesite and their respective flow laws. Experiments using fine-grained magnesite were performed in a Heard gas confining medium rock deformation apparatus at a constant effective pressure (= confining pressure-CO2 pressure) of 300MPa. Experiments using coarse-grained magnesite were performed in a Griggs piston-cylinder rock deformation apparatus at a constant effective pressure of 900MPa. At low temperatures (T<600°C, strain rate = 10-5/s) both magnesite aggregates deform by crystal plastic mechanisms predominated by dislocation glide. At higher temperatures the coarse-grained magnesite deforms by dislocation creep and the fine-grained magnesite deforms by diffusion creep. The strain rate and temperature dependence of the strength of magnesite deforming by low-temperature plasticity, dislocation creep, and diffusion creep can be described by power law flow laws with stress exponents (n) of 19.7, 3.0, and 1.1 and activation enthalpies of 229, 410, and 209 kJ/mol, respectively. The strength of the low-temperature plasticity data can also be described using an exponential flow law with a=0.022MPa-1 with an activation enthalpy of 233 kJ/mol. Extrapolation of the flow laws to natural conditions indicates that magnesite is generally stronger than calcite and dolomite assuming similar grain sizes. However, its strength is orders of magnitude lower than olivine at all conditions in the Earth's mantle and may promote deep-focus earthquakes through ductile instabilities.