General Director Gagik Markosyan said the plant's sole working reactor was started up without difficulty on Wednesday after being refueled with US$14.5 million (Ђ12.4 million) in nuclear fuel supplied by Russia.
Immediately after startup, Armenian security forces conducted an exercise at the plant to train against a hypothetical terrorist threat, he said.
The aging, Soviet-built plant was closed after a devastating 1988 earthquake, but returned to service in 1995 during a severe energy shortage.
Armenia has been under international pressure from the European Union to shut the plant down due to safety concerns, but officials are resisting the call, as the plant supplies nearly 40 percent of the nation's electricity.
President Robert Kocharian said last month that the country intended to build a new nuclear plant, reported AP.