We have performed sum frequency generation (SFG) spectroscopy of water molecules on a polycarbonate (PC) film with and without 1 minute O2 plasma exposure. The intensity of SFG signal corresponding to the O-H stretch vibrations of water at a water/PC film interface was ∼1000 times as low as that of signal from a water/quartz interface. In the SFG spectrum of water facing a pristine PC film, peaks at 3270 and 3440 cm-1, and a shoulder at 3620 cm-1 were observed. In the SFG spectrum of water on the film exposed to O2 plasma, peaks were observed at 3200 and 3450 cm-1. The peak at 3270 cm-1 red-shifted to 3200 cm-1 by the O2 plasma exposure. The intensity of the peak at 3200 cm-1 was higher than that at 3450 cm-1. The shoulder at 3620 cm-1 disappeared after the O2 plasma exposure. The modulations of these peaks represent that the packing structure of water molecules was more ordered on the film after the O2 plasma exposure.