Washington accuses the Islamic republic of using a civilian nuclear program to hide a quest for atomic weapons, and has charged that Iran is a destabilizing force in Iraq. Tehran has denied that it seeks nuclear arms.
"I called it (Iran) part of the 'axis of evil' for a reason," Bush said in an interview with Fox News. "It's a real threat."
The US president first lumped Iran with North Korea and Saddam Hussein's Iraq in an "axis of evil" during his 2002 State of the Union speech to the US Congress.
His comments Wednesday came amid an escalating war of words with Tehran, whose hardline President Mahmoud Ahmadinejad has stoked outrage in the West by calling Israel a "tumor" in the Middle East and saying that the Holocaust is a myth.
Asked how he would deal with Iran, Bush replied: "We continue to work the diplomatic front."
"I'm concerned about theocracy that has got little transparency, a country whose president has declared the destruction of Israel as part of their foreign policy, and a country that will not listen to the demands of the free world to get rid of its ambitions to have a nuclear weapon," said Bush.
Asked whether he had a message for authorities in Tehran, Bush said: "I would hope they'd be wise enough to begin to listen to the people and allow the people to participate in their government."