  Clamping loss limits the quality factor of mechanical mode in the
optomechanical resonators supported with the supporting stem. Using the
mechanical bound state in the continuum, we have found that the mechanical
clamping loss can be avoided. The mechanical quality factor of microsphere
could be achieved up to 10^8 for a specific radius of the stem, where the
different coupling channels between the resonator and supporting stem are
orthogonal to each other. Such mechanism is proved to be universal for
different geometries and materials, thus can also be generalized to design the
high quality mechanical resonators.
