  We have studied superfluid-Mott insulating transition of spin-1 bosons
interacting antiferromagnetically in an optical lattice. We have obtained the
zero-temperature phase diagram by a mean-field approximation and have found
that the superfluid phase is to be a polar state as a usual trapped spin-1 Bose
gas. More interestingly, we have found that the Mott-insulating phase is
strongly stabilized only when the number of atoms per site is even.
