Superheater drain tube is one of the components of Heat Recovery Steam Generator (HRSG). The tube experienced failure after twenty years of operation. The failure was noticed due to the finding of water drips during regular inspection. After removal of pipe insulation during operation, it was found that there was a steam leakage in the tube. The paper analyzes the failure of the superheater drain tube so that similar failure can be avoided. Field observation, visual inspection, XRD, chemical test and metallography were carried out in order to investigate the failure. Results of field observation and visual inspection reveal that the pipe experienced wall thinning originated from external side of the pipe. No anomalies or defects can be found in the internal wall of the pipe. Hence, it is determined that the tube leakage was caused by progressive external or outer wall thinning due to corrosion under insulation. The corrosion occurs due to reaction between tube external surface and water, forming corrosion product in the form of Fe3O4 and Fe2O3. The source of the water may originate from water moisture yielded from condensation of tube surface and water contained glass wool insulation. Hanging clamp improper installation may also act as water pathway from external environment into the glass wool.