The 300MW turbine generator which uses Tung oil-BMI-epoxy resin-rich insulation in Wu Jing Thermal Power Plant was subjected to a wide variety of diagnostic tests during the regular planned overhaul. This generator has kept safely running for 23 years since 1991. Diagnostic tests included direct current diagnostic tests, capacitance, dissipation factor and tip-up, partial discharge, AC proof voltage test. Meanwhile, a new generator (reference generator) with the same structure was tested. The aging degradation of generator stator insulation is assessed by comparison the data of diagnostic tests for two generators. It is concluded that the aging situation of the candidate generator stator insulation is not serious and still meets the requirements of operation after safely running 23 years.