  We numerically study the interplay of band structure, topological invariant
and disorder effect in two-dimensional electron system of graphene in a
magnetic field. Two \emph{distinct} quantum Hall effect (QHE) regimes exist in
the energy band with the unconventional "half-integer" QHE appearing near the
band center, consistent with the experimental observation. The latter is more
robust against disorder scattering than the conventional QHE states near the
band edges. The phase diagram for the unconventional QHE is obtained where the
destruction of the Hall plateaus at strong disorder is through the float-up of
extended levels toward band center and higher plateaus always disappear first.
We further predict a new insulating phase between $\nu =\pm 2$ QHE states at
the band center, which may explain the experimentally observed resistance
discontinuity near zero gate voltage.
