  We provide a semiquantitative explanation of a recent experiment on the
resonant inelastic x-ray scattering (RIXS) in insulating cuprate La2CuO4 (Y.J.
Kim et al., Phys. Rev. Lett. 89, 177003 (2002).). We show theoretically that
there are three characteristic peaks in RIXS spectra, two of which are
attributed to the charge transfer excitation and are reasonably assigned to
those observed experimentally. The lowest energy peak has a relatively large
dispersion ($\sim 0.8$ eV) and is suppressed near the zone corner (\pi, \pi),
in agreement with the experiment. We stress that electron correlation is an
essential factor for explaining the overall energy-momentum dependence of the
RIXS spectra consistently.
