State-run television announced the blog's launch on Sunday, urging the public to write messages to the president via the website.
In his first entry Ahmadinejad wrote about his childhood, the country's Islamic revolution and Iran's war with Iraq.
The blog also asks readers to participate in an online poll which asks if they think the United States and Israel are "pulling the trigger for another world war".
The blog is translated into English, French and Arabic.
Ahmadinejad wrote about how Ayatollah Khomeini began to appeal to him when the ayatollah was in exile during the 1960s and 70s.
He wrote: "The more I became familiar with his thought and philosophy, the more affection I had for that divine leader and his separation and absence was intolerable for me."
He also used his blog entry to criticise the United States and to reflect upon the 1980-88 war with Iraq, when he served in the Revolutionary Guard.
He called Saddam Hussein, the former Iraqi president, the "aggressor" and wrote that international organisations tried to "distort and hide the facts that Saddam was the aggressor and that the arrogant powers had fully supported him".
The blog was 2,000 words long in English and dated Friday. At the end of the blog, Ahmadinejad wrote: "From now onwards, I will try to make it shorter and simpler."
Ahmadinejad's government has arrested and placed pressure on some bloggers as part of a wider internet clampdown begun after he became president last year.
Mohammad Ali Dadkhah, a prominent human rights lawyer, estimates that at least 50 bloggers have been detained since last year.
Iranian bloggers first started proliferating about five years ago.
Isa Saharkhiz, a member of the Iranian branch of the committee to protect journalists, said: "The crackdown on bloggers is part of a growing censorship policy by the state."