We studied the effects of indium codoping on the moisture stability of 50- and 100-nm-thick Ga-doped ZnO (GZO) films grown at various oxygen gas flow rates (OFRs) by ion plating with direct-current arc discharge. The results of damp-heat tests at a temperature of 60 degrees C and a relative humidity of 95% for 500 h showed that, for 100-nm-thick GZO film with 0.75 wt% In2O3 codoping (GZO:In) grown at an OFR of higher than 10 sccm, the relative change in resistivity (Ap) was greatly reduced to be less than 10%. Furthermore, Delta rho = 7.4% was achieved for 50-nm-thick GZO:In films grown at an OFR of 15 sccm. Delta rho is defined as Delta rho = 100% x vertical bar rho-rho(0)vertical bar/rho(0), where rho(0) and rho are electrical resistivities before and after the damp-heat tests, respectively. The low resistivity and high transparency of GZO films were retained by indium codoping. No phase segregation of Ga2O3 and In2O3 in the GZO:In films was observed by X-ray diffraction and energy-dispersive X-ray spectroscopy in scanning transmission electron microscopy. The water diffusion into GZO film during damp-heat test was blocked in GZO:In film grown at OFR = 15 sccm, which was directly observed by secondary ion mass spectroscopy. All these findings demonstrate that indium codoping is an effective method of improving the moisture stability of thin GZO film of thickness less than 100 nm. This is essential for applications including optoelectronic devices.