Chavez said the fund "will permit us to underpin investments ... above all in those countries whose governments are making efforts to liberate themselves from the [US] imperialist yoke ... Death to US imperialism."
Ahmadinejad, who is on a tour of Latin America, said that Tehran and Caracas had the task of "promoting revolutionary thought in the world".
"The reason for all the current problems is the erroneous direction of the powerful countries, where there is poverty, hate, enmity and war," he added.
The two presidents announced that they would make a joint effort to obtain new oil production cuts.
"Today we know that there is too much crude in the market, that's why we support ... the decisions that have been taken to reduce production and protect the price of oil," Chavez said.
He emphasised that he was sending the message "to all the heads of state in the Opec countries to continue to strengthen our organisation in this direction".
Members of the 11-nation Organisation of Petroleum Exporting Countries (Opec) have expressed concern about the falling price of oil, which has slid 14 per cent since the start of the year.
Ahmadinejad has praised Chavez for his outspoken support of Iran's nuclear programme, which the US and European governments say may be part of a project to build atomic weapons.
Facing the threat of international isolation and sanctions imposed by the UN Security Council over its uranium enrichment work, Iran is keen to demonstrate it has backing among a number of leaders in Latin America.
Chavez is the most vocal supporter in Latin America for Iran and its president, with both men calling each other "brother" and relishing their status as fierce opponents of Washington's influence.
"Hugo is my brother," Ahmadinejad said during his last visit to Venezuela in September. "Hugo is the champion of the fight against imperialism."
In September 2005, Venezuela was alone in opposing a resolution at the International Atomic Energy Agency (IAEA) that found Iran in violation of nuclear safeguards. Chavez has since backed Iran's right to enrich uranium.
Iran and Venezuela are both important players in Opec and have signed numerous co-operation agreements in the energy sector and other fields.
During a visit to Iran last September, Chavez came out in support of Iran's nuclear programme, as well as denouncing Israeli military operations in Lebanon.
The two presidents also signed deals covering iron and steel production, petrochemicals, pharmaceuticals and health care equipment and munitions.
While Ahmadinejad seeks to cultivate Latin American allies, Condoleezza Rice, the US secretary of state, is in the Middle East to rally Arab support for a new US strategy in Iraq and counter Iran's alleged "interference" in Iraq.
Ahmadinejad arrived in Nicaragua late on Saturday, where Daniel Ortega has just returned to power. The Iranian president was met at the airport by the new Nicaraguan president.
On Monday, Ahmadinejad will take part in the swearing-in ceremony of Ecuador's new president Rafael Correa, who has vowed to forge stronger ties with Venezuela and not to renew a lease for a US military air base on the country's Pacific coast.
The Iranian president will also hold meetings with other South American presidents including Bolivia's Evo Morales on the sidelines of the ceremony in Ecuador, before finishing his tour on Tuesday.