Charge ordering of a mixed valence system LuFe2O4 has been studied by neutron scattering technique, focusing attention on the charge frustration effect on triangular lattice. It is revealed that the system undergoes successive phase transitions following the sequence of disorder → two-dimensional charge density wave state (2D-CDW) → three-dimensional charge density wave state (3D-CDW). In 2D-CDW state, the charge density wave defined by the wave vector (1/3 1/3) is formed in the hexagonal layer, © 1998 Elsevier Science B.V. All rights reserved.