Si-Ge interdiffusion and strain relaxation were studied in a metastable SiGe epitaxial structure. With Ge concentration profiling and ex-situ strain analysis, it was shown that during thermal anneals, both Si-Ge interdiffusion and strain relaxation occurred. Furthermore, the time evolutions of both strain relaxation and interdiffusion were characterized. It showed that during the ramp-up stage of thermal anneals at higher temperatures (800°C and 840°C), the degree of relaxation, R, reached a “plateau”, while interdiffusion was negligible. With the approximation that the R value is constant after the ramp-up stage, a quantitative interdiffusivity model was built to account for both the effect of strain relaxation and the impact of the relaxation induced dislocations, which gave good agreement with the experiment data.