IEC/TR 62627-03-03@ which is a technical report@ describes the investigation results of highpower reliability for metal-doped optical fibre plug-style attenuators. This report contains the high-power test results for metal-doped optical fibre SC plug-style optical attenuators@ the thermal simulation results and the analysis of degradation modes@ long-term reliability test results under high-power conditions and the derivation of maximum limit of optical power for guaranteeing long-term operation.