SiGe quanrum rings (QRs) grown at 500°C and 600°C were observed on SiGe quantum dots (QDs) capped with Si. Average depth and diameter are 9 nm and 185 nm, respectively, for QRs at 500°C, while those are 0.9 nm and 84 nm for QRs at 600°C. Ge out-diffusion mechanism is proposed to be responsible for nanorings formation at 500°C, and Si surface diffusion toward strain-free edges is proposed to be responsible for nanorings formation at 600°C. Raman spectroscopy suggests that formation of QRs at 600°C is closely correlated with a strain-driven process. QRs grown at 600°C are the metastable states and can be only observed in very limited conditions. Both thick cap and high thermal budget can destroy SiGe nanorings structures.