Mahathir, who will turn 82 in December, was admitted to the National Heart Institute on Sunday to prepare for his second coronary bypass surgery.
Yahya Awang, the surgeon who performed Mahathir's first bypass in 1989, heads the surgery team in the operation which began early Tuesday and is due to last up to six hours, said Mahathir's special officer Sufi Yusoff.
In November, the fiery and outspoken Mahathir suffered a mild heart attack, followed by bouts of lung infection and breathing difficulties earlier this year.
His daughter, Marina, wrote on her Web journal Monday that the former leader would be the oldest person in the country to have a second bypass, but expressed her confidence in the team of specialists tending to her father.
"Two weeks ago, he went in for some tests and an angiogram to determine the next course of action after which this decision to redo the bypass was made," Marina wrote.
Mahathir led Malaysia for 22 years until 2003, and stepped down after choosing his then deputy, Abdullah Ahmad Badawi, to replace him.
Last year, Mahathir began launched scathing attacks on Abdullah and his government, accusing the new leader of corruption, incompetence and mismanaging the economy.
However, his bout of illnesses and frequent admissions to hospital curtailed his vocal oppositions to the government after doctors and family members advised him to slow down and rest more.