We have conducted uniaxial compression experiments on fine-grained diopside with either 4 or 10 vol% forsterite and 10 vol% anorthite. We changed the average grain size in the diopside with forsterite aggregate from 0.43 to 4.07µm to reveal the grain size sensitivity of the creep rate. Mechanical data were obtained at a stepped load for a temperature range of 1050°C–1170°C. The observed strain rates are best explained by stress and grain size exponent values of 1 and 2, respectively, and activation energy of 720kJ/mol, which indicates lattice diffusion creep. Anorthite bearing aggregate is 3 times weaker than forsterite-bearing aggregate, probably due to the presence of aluminum. We have compared previously reported diffusion creep rates of diopside in nondimensional stress and strain-rate space, constructed based on our diffusion creep law. It demonstrates that all the earlier mechanical data are well summarized by our diffusion creep flow law