This paper presents evaluating procedure for engine generator operated over 13 years which are usually installed at the island and therefore, isolated to the main distribution line. To decide continual operation of the generator, their diagnosis results should guarantee safety running. So the determination of criteria for continual usage or replacement is essential for economical and reliable maintenance. We present all the courses of evaluating the condition which includes the processes from acquiring basic data to comprehensive determination of generators. All the test was applied to serial number 3 generator in Jodo Engine Power Plant.