The two leaders met this week when they attended the United Nations General Assembly in New York.
Lula told the O Globo newspaper, "I have nothing to do with what other presidents think. I do not know whether they like him (Ahmadinejad) or not. I am meeting with him because he is the president of a great nation and we talk as heads of state. This is not a friends' club." Lula is scheduled to visit Iran in May 2010.
He said that Ahmadinejad had assured him that Iran's nuclear program only sought peaceful goals, MercoPress reported.
"I do not take rumors into account ," Lula told O Globo. "So far there is nothing that can serve to reject the conclusion that Iran wants (nuclear) technology for peaceful ends."
Last year, Lula unveiled plans for building up a nuclear capacity in Brazil for civilian and scientific purposes. He is talking with France on developing a nuclear-powered submarine in Brazil with French help. Brazil's nuclear program dates back to the 1930s and was developed, mostly with U.S. help, in response to Argentina's nuclear projects.
The Brazilian leader's interview ranged from nuclear issues to Iran's president elections and oil and trade collaboration.
Amid international revelations Friday of a "secret" Iranian nuclear plant -- secret to everyone except the intelligence agencies -- Lula advised Ahmadinejad to allow international supervision of Iran's nuclear program. He warned that "international sanctions can be painful and distressing."
Lula appeared to dismiss protests against Iran's presidential elections that brought Ahmadinejad back into power.
Lula said Iranians had the right to protest the election results, pointing out, "In Brazil we also have people who do not accept electoral defeats. I lost three elections before I won my first one, and I was OK with that. In Iran, 85 percent of the people voted."
He said the Iranian opposition leaders can protest as much as they want, "but what right do I have as president of Brazil to question election results in Iran?" MercoPress reported.
Lula said he felt no constraints about meeting with Ahmadinejad amid continuing controversies over his election victory and pronouncements. "Iran is a leading global producer of oil, so we have much to talk about," he said. A Brazilian trade delegation will visit Iran shortly.
Earlier this month, Venezuelan President Hugo Chavez visited Tehran and hailed Iran as a strategic partner.
Analysts noted comparisons between Brazil and Iran in both hydrocarbon and nuclear programs. Iran has been running its oil and gas industry without major foreign help since the 1979 Islamic take-over, while Brazil's state-run Petrobras has emerged as one of the largest independent energy companies in the world.
Both Brazil and Iran began their nuclear programs with Western encouragement. In Iran's case, the major contenders for the nuclear development program under the Shah were all Western companies, including major U.S., French and German nuclear industry suppliers.
The program went on the back burner when Ayatollah Ruhollah Khomeini displaced the Shah in 1979, forcing him into exile, and installed the Islamic Republic of Iran.