  In this letter we show that the vortex lattice structure in the Bose-Fermi
superfluid mixture can undergo a sequence of structure transitions when the
Fermi superfluid is tuned from the BCS regime to the BEC regime. This is due to
different vortex core structure of the Fermi superfluid in the BCS regime and
in the BEC regime. In the former the vortex core is nearly filled, while the
density at the vortex core gradually decreases until it empties out at the BEC
regime. Therefore, with the density-density interaction between the Bose and
the Fermi superfluids, the two sets of vortex lattices interact stronger in the
BEC regime that yields the structure transition of vortex lattices. In view of
recent realization of this superfluid mixture and vortices therein, our
theoretical predication can be verified experimentally in near future.
