Recent works revealed that a ferroelectric polarization in the charge-frustrated system LuFe₂O₄ originates from an emergence of the charge ordering, which is characterized by a regular arrangement of Fe²⁺ and Fe³⁺ ions on the frustrated triangular lattice. In this work, we have investi-gated both the charge ordered (CO) structure and charge ordering processes mainly by means of the transmission electron microscopy (TEM) . Both the CO structure and charge ordering processes depend strongly on the amount of the oxygen deficiency (δ) involved in LuFe₂O_4-δ. In the case of δ-0, the three-dimensional CO structure with the modulation vector of q= [1/3 1/3 1/2] was formed at room temperature. On the other hand, as δ was increased, the three-dimensional CO structure changed into the two-dimensional CO one and the coherent length becomes shorter along the [001] direction. For proper understanding of the dielectric and magnetic properties in LuFe₂O₄ and related materials, it should be essential to prepare LuFe₂O_4-δ samples with properly controlled oxygen-concentration and elucidate the physical properties in LuFe₂O_4-δ with δ-0.