We demonstrate radiation-pressure-driven mechanical oscillations from high optical quality factor silica microdisk resonators on chip. Mechanical quality factors of 3520 in air and 12540 in vacuum for the fundamental radial breathing modes are obtained from 73 μm-diameter silica microdisks with mechanical frequencies of ~51 MHz. The measured mechanical oscillation threshold powers for the input light are determined to be 62.5 μW in air and down to 26.6 μW in vacuum. We demonstrate that radiation-pressure-driven mechanical oscillations from high optical quality factor silica microdisk resonators on chip. Mechanical quality factors of 3520 in air and 12540 in vacuum for the fundamental radial breathing modes are obtained from 73 μm-diameter silica microdisks with mechanical frequencies of ~ 51 MHz. The measured mechanical oscillation threshold powers for the input light were determined to be 62.5 μW in air and down to 26.6 μW in vacuum.