Mechanisms of photoresist removal were investigated using microwave-excited plasma in saturated water vapor under a reduced pressure condition (approx. 6 kPa). A few seconds of white emission were observed at the beginning of photoresist removal. To ascertain the species of spectra, optical emission spectroscopy was used for photoresist removal from a Si wafer. Emission spectra of OH (A2Σ – X2Π), Hα (656 nm), and O (777 nm) derived from dissociation of water molecules were observed. Other emission spectra attributed to CO molecules were also detected and identified as the Ångström system (B1Σ – A1Π). This result indicates CO molecule generation during photoresist removal. The CO molecule emission intensity reached a maximum value at 1.1 s after plasma ignition, whereas those of OH, Hα, and O reached a minimum value at almost identical time. Furthermore, normalized emission spectra shape from CO molecules during photoresist removal were almost identical for ∆v = +1, 0,-1,-2,-3, and-4. Rotational and vibrational temperatures of the CO molecules during photoresist removal were found to be almost constant and to be 1500 and 2500 K, respectively. These results suggest increased CO molecular density above the Si wafer at 1.1 s.