A double-low-temperature-buffer variable-temperature growth scheme was studied for fabrication of strain-relaxed thin Si0.6Ge0.4 layer on Si(001) by using molecular beam epitaxy (MBE), with particular focuses on the influence of growth temperature of individual low-temperature-buffer layers on the relaxation process and final structural qualities. The low-temperature buffers consisted of a 40 nm Si layer grown at an optimized temperature of similar to 400 degrees C, followed by a 20 nm Si0.6Ge0.4 layer grown at temperatures ranging from 50 to 550 degrees C. A significant relaxation increase together with a surface roughness decrease both by a factor of similar to 2, accompanied with the cross-hatch/cross-hatch-free surface morphology transition, took place for the sample containing a low-temperature Si0.6Ge0.4 layer that was grown at similar to 200 degrees C. This dramatic change was explained by the association with a certain onset stage of the ordered/disordered growth transition during the low-temperature MBE, where the high density of misfit dislocation segments generated near surface cusps largely facilitated the strain relaxation of the top Si0.6Ge0.4 layer.