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Low-temperature scanning tunneling microscopy measurements have been performed on YBa 2 Cu 3 O 7Ϫ␦ single crystals with various oxygen contents (6.65р7Ϫ␦р6.98). In the CuO-chain layer, we observe the doping-dependent charge modulation images that are attributed to the charge-density wave ͑CDW͒ due to the 2k F instability of the one-dimensional Fermi surface. Even in the so-called 60-K T c plateau region, the CDW correlation in the CuO-chain layer survives partially. Our scanning tunneling microscopy results indicate that the CuO chain without oxygen vacancies is 1/4 filled and prefers to form a CDW state.