We report a Cu K-edge resonant inelastic X-ray scattering (RIXS) study of high-Tc cuprates. Momentum-resolved charge excitations in the CuO2 plane are examined from parent Mott insulators to carrier-doped superconductors. The Mott gap excitation in undoped insulators is found to commonly show a larger dispersion along the [π, π] direction than the [π, 0] direction. On the other hand, the resonance condition displays material dependence. The Mott gap persists in carrier-doped states. Upon hole doping, the dispersion of the Mott gap excitation becomes weaker associated with the reduction of antiferromagnetic correlation and an intraband excitation appears as a continuum intensity below the gap at the same time. In the case of electron doping, the Mott gap excitation is prominent at the zone center and a dispersive intraband excitation is observed at finite momentum transfer. © 2008 Elsevier Ltd. All rights reserved.