A novel on-line monitoring method for the early detection of overheating in turbine generators has been developed. This paper deals with the gas sampling procedure, the gas-phase concentration changes of the organic species in the turbine generator, and theoretical discussion about overheating-detection.The concentration of the gas-phase organic species in an operating generator has been monitored over a period of 1 year. The results revealed that the background concentration increased for ahout 100 days at the beginning of the operation and then slowly decreased. Bisphenol-A and some molecules were recognized as characteristic decomposition indicators from epoxy. It was theoretically predicted that the slope of rapid concentration increase is corresponding to the overheating temperature and the volume. Detecting the overheating problem is considered feasible by tracing relative concentration gradient of the decomposition species from the materials.