The critical thickness of tensile-strained-Si layers grown at 600 °C on (001) surfaces of silicon-germanium-on-insulator substrates is experimentally studied for a tensile strain range between 0.4% and 1.2% and is found to be rather thinner than Houghton’s critical thickness [J. Appl. Phys. 70, 2136 (1991)] for compressively mismatched SiGe/Si (001) system. The thinner critical thickness in tensile-strained Si is attributed to the fact that the stress-relieving misfit dislocations are 90° partial dislocations, instead of 60° misfit dislocations for compressive SiGe layers on Si (001) substrates. Although 60° misfit dislocation is found to be increasingly formed with increasing strained-Si layer thickness, the strain relaxation is sluggish and the tensile strain mostly remains for supercritical thickness ten times thicker than the critical thickness.