  We develop a chip-scale cavity optomechanical system in single-crystal
lithium niobate that exhibits high optical quality factors and a large
frequency-quality product as high as $3.6\times 10^{12}$ Hz at room temperature
and atmosphere. The excellent optical and mechanical properties together with
the strong optomechanical coupling allow us to efficiently excite the coherent
regenerative optomechanical oscillation operating at 375.8 MHz with a threshold
power of 174 ${\rm \mu W}$ in the air. The demonstrated lithium niobate
optomechanical device enables great potential for achieving
electro-optic-mechanical hybrid systems for broad applications in sensing,
metrology, and quantum physics.
