  We study superfluidity of supersolid phases of dipolar Bose gases in
two-dimensional optical lattices. We perform linear stability analyses for the
corresponding dipolar Bose-Hubbard model in the hardcore boson limit to show
that a supersolid can have stable superflow until the flow velocity reaches a
certain critical value. The critical velocity for the supersolid is found to be
significantly smaller than that for a conventional superfluid phase. We propose
that the critical velocity can be used as a signature to identify the
superfluidity of the supersolid phase in experiment.
