By Pam O'Toole
BBC News, Tehran
Iranian President Mahmoud Ahmadinejad has replaced his key oil and mines and industry ministers.
Mr Vaziri-Hamaneh was the president's fourth choice for the role
Mr Ahmadinejad's press advisor, Ali Akbar Javanfekr, said it was necessary to make changes to meet the interests of the people and improve things.
On Sunday, Iran's state media announced the ministers' resignations.
Iran's press has speculated that Mr Ahmadinejad might be seeking to bring the oil ministry more closely under his own control.
Outgoing Oil Minister Kazem Vaziri-Hamaneh was appointed two years ago.
A technocrat, he was Mr Ahmadinejad's fourth choice for the post, approved only after the president's first three candidates were rejected by the conservative-dominated parliament.
The post of oil minister is a crucial one; oil makes up 80% of Iran's export earnings.
Petrol rationing policies sparked riots across Iran in June
There has been much speculation in Iran's media about why he stepped down - some newspapers or websites suggest there might have been differences between Mr Vaziri-Hamaneh and the president over proposed personnel changes in the ministry.
Others suggest that he may have been replaced because he had not attracted foreign investment, because of the introduction of petrol rationing or because of his handling of a controversial gas deal between Iran, India and Pakistan.
There has also been speculation that Mr Ahmadinejad, who took office on a platform of trying to tackle what he called "oil mafias" and redistributing the country's wealth, might be seeking to bring the oil ministry more closely under his own control.
Several moderate or reformist newspapers alleged the president had failed to live up to his economic promises and that ordinary people were dissatisfied because living standards had fallen.
Some analysts suggest that the replacement of Mr Vaziri-Hamaneh and Mines and Industry Minister Ali Reza Tahmasebi could be a way of diverting attention from this, but Mr Javanfekr told Iran's official news agencies the ministers had resigned out of necessity.
He said their replacement showed the government was vigorous and moving ahead.