  Anderson localization of electron states on graphene lattice with diagonal
and off-diagonal (OD) disorder in the absence of magnetic field is investigated
by using the standard finite-size scaling analysis. In the presence of diagonal
disorder all states are localized as predicted by the scaling theory for
two-dimensional systems. In the case of OD disorder, the states at the Dirac
point (E=0) are shown to be delocalized due to the specific chiral symmetry,
although other states ($E \neq 0$) are still localized. In OD disorder the
conductance at E=0 in an $M\times L$ rectangular system at the thermodynamical
limit is calculated with the transfer-matrix technique for various values of
ratio $M/L$ and different types of distribution functions of the OD elements
$t_{nn'}$. It is found that if all the $t_{nn'}$'s are positive the conductance
is independent of $L/M$ as restricted by 2 delocalized channels at E=0. If the
distribution function includes the sign randomness of elements $t_{nn'}$, the
conductivity, rather than the conductance, becomes $L/M$ independent. The
calculated value of the conductivity is around $\frac{4e^2}{h}$, in consistence
with the experiments.
