We investigated the etch characteristics of chromium films by using Cl-2/O-2 gas mixtures with electron cyclotron resonance plasma. In order to examine the chemical etch characteristics of Cl-2/O-2 gas plasma, we examined the etch rate with various gas mixing ratios. By X-ray photoelectron spectroscopy (XPS), the surface reaction on the chromium films during the etch was examined. At the same time, the plasma characteristics were examined by optical emission spectroscopy. From XPS analyses, it was confirmed that a chromium oxychloride (CrClxOy) layer was formed on the surface by the etch using Cl-2/O-2 gas mixtures. We observed a new characteristic emission line at 517 nm during the etch of chromium films with Cl-2/O-2 gas mixtures, using optical emission spectroscopy. It was found that the peak intensity of this emission line had a similar behavior to the etch rate. It was proposed that the emission line of 517 nm should result from chromium compounds with the oxidation state of Cr3+.