We report the results of a theoretical analysis for the kinetics of strain relaxation in Si1-xGex thin films grown epitaxially on Si(100) substrates. The analysis is based on a properly parametrized dislocation mean-field theoretical model describing plastic deformation dynamics due to threading dislocation propagation and addresses strain relaxation kinetics during both epitaxial growth and thermal annealing, including post-implantation annealing. Theoretical predictions for strain relaxation as a function of film thickness in Si0.80Ge0.20/Si(100) samples annealed after epitaxial growth either unimplanted or after He ion implantation are in excellent agreement with experimental measurements [J. Cai etal, J. Appl. Phys. 95, 5347 (2004)].