Si-capping-induced surface roughening, accompanying Si-Ge alloying, is reported for strip structures of Ge selectively grown on Si via ultrahigh vacuum chemical vapor deposition. A 0.7-μm-wide strip structure of Ge running in the [110] direction, as well as a 100-μm-wide mesa structure, is selectively grown on an Si (001) surface exposed in an SiO2-masked Si substrate. In contrast to a wide mesa structure with a Ge thickness of 0.5 μm, composed of a (001) plane at the top and {113} facet planes at the sidewalls, the (001) top plane almost disappears for the narrow strip structure. The strip is mainly surrounded with inclined {113} planes near the top and adjacent {111} planes at the side, while the structure near the bottom edges depends on the growth temperature (600/700 °C). An Si cap layer with a thickness of 10 nm or larger is subsequently grown at 600 °C to protect the fragile Ge surface. The scanning electron microscopy observations reveal a roughened surface on the {113} planes, with depressions specifically induced near the boundary with the {111} planes. The Raman spectra indicate that an SiGe alloy is formed on the strip and the wide mesa sidewalls due to the Si-Ge interdiffusion. There is no such SiGe alloy on the (001) plane of the wide mesa top. The Si cap layer with a misfit strain probably works as a stressor for the underlying Ge, applying stress concentrated around the facet boundaries and inducing a mass transport alongside the Si-Ge interdiffusion for strain relaxation. In terms of the fabrication of practical devices, it is important to suppress the roughening and alloying significantly by decreasing the growth temperature for the Si cap layer from 600 to 530 °C.