  We show, by direct computations of bosonic string spectra, the O(d,d;Z)
(d=1,2) T-duality in the maximally supersymmetric IIB plane-wave background
compactified on S^1 and T^2. Only half of the ordinary set of zero modes appear
in the Hamiltonian. This "half" Narain lattice is proved to be stabilized by
the T-duality group.
