Er2O3 is one promising material for use as an insulating coating in a liquid lithium blanket system. In this study, corrosion of bulk Er2O3 in Li with high or low O concentrations was investigated at 600°C. The tests revealed that the corrosion reaction is accelerated under high O condition and suppressed in low O concentration. From these results we assumed the corrosion reaction and tested with more precise condition. The Er2O3 bulk specimen corroded significantly in Li with more than 300 ppm O. Control of O concentration is essential for a Li blanket system with an Er2O3 coating.