  We numerically study the quantum Hall effect in biased bilayer graphene based
on a tight-binding model in the presence of disorder. Integer quantum Hall
plateaus with quantized conductivity $\sigma_{xy}=\nu e^2/h$ (where $\nu$ is
any integer) are observed around the band center due to the split of the valley
degeneracy by an opposite voltage bias added to the two layers. The central
($n=0$) Dirac Landau level is also split, which leads to a pronounced $\nu=0$
plateau. This is consistent with the opening of a sizable gap between the
valence and conduction bands. The exact spectrum in an open system further
reveals that there are no conducting edge states near zero energy, indicating
an insulator state with zero conductance. Consequently, the resistivity should
diverge at Dirac point. Interestingly, the $\nu=0$ insulating state can be
destroyed by disorder scattering with intermediate strength, where a metallic
region is observed near zero energy. In the strong disorder regime, the Hall
plateaus with nonzero $\nu$ are destroyed due to the float-up of extended
levels toward the band center and higher plateaus disappear first.
