As Q factor is boosted in microscale optical resonant systems there will be a natural tendency for these systems to experience a radiation-pressure induced instability. The instability is manifested as a regenerative oscillation (at radio frequencies) of the mechanical modes of the microcavity. The first observation of this radiation-pressure-induced instability is reported here. Embodied within a microscale, chip-based device reported here this mechanism can benefit both research into macroscale quantum mechanical phenomena and improve the understanding of the mechanism within the context of LIGO. It also suggests that new technologies are possible which will leverage the phenomenon within photonics.