Residual strain and surface roughness of Si1-xGex alloy layers grown by molecular beam epitaxy on Si(001) substrate at 550 degrees C have been characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM). Two kinds of samples were grown. One is a series of the Si1-xGex alloy layers with x less than or equal to 0.3 and thickness of 5000 Angstrom directly grown on Si(001) substrate, and the other is the Si0.7Ge0.3 alloy layers with thickness of 2000 Angstrom grown on Si(001) via compositionally graded Si1-yGey (0 less than or equal to y less than or equal to x) buffer layers. The Ge grading rate in the buffer layer, defined as % Ge/mu m, was ranged from 22 to 76. In the case of direct growth, the surface morphology changes from wavy ripple pattern to a cross-hatch pattern with increase in x, and island-like pattern appears at x = 0.3. The residual strain decreases with increase in x, whereas the surface roughness increases with x. In the case of Si0.7Ge0.3 alloy layers grown with buffer layers, the surfaces of all samples display crosshatch pattern. The surface roughness shows maximum for a grading rate of about 35, and it decreases for both lower and higher grading rates. The residual strain also shows a similar dependence on the grading rate.