Corrosion of candidate materials of insulating coating was investigated in natural convection lithium (Li) loop. The loop was constructed for present study and bulk specimens of Er2O3, Y2O3 and AlN were placed at several positions in the loop. The operation temperature was 400 °C in the low temperature region and 500 °C in the high temperature region. After the exposure, specimens were cleaned and the corrosion characteristics were investigated. The results indicated that the corrosion characteristics of the bulk specimens were similar to those obtained by the static test in the previous study.