Micro-disk resonators with high Q-factor have been experimentally demonstrated on germanium-on-insulator (GOI). GOI substrates fabricated by direct wafer bonding show better crystal quality that germanium films directly grown on Si. Sharp resonant peaks with Q-factor around 1000–4000 have been observed from micro-disks fabricated on GOI substrate by low-temperature photoluminescence measurements. The light emission properties against pump laser power and device temperature are also investigated. Our results indicating that GOI micro-disks are promising resonators for low threshold, ultra-compact Ge lasers on Si.