Mittal, whose fortune is estimated at 27 billion pounds, already owns two large homes in Kensington Palace Gardens where Princess Diana spent her last years.
The latest purchase of the former embassy of the Philippines from Hedge fund tycoon Noam Gottesman came a month after Mittal bought Britain's most expensive house for 117 million pounds on the street nicknamed "Billionaire's Row", ostensibly for his son Aditya and his family.
The 70 million pounds price tag for Mittal's latest property is all the more remarkable as the former Philippine Embassy is in need of modernisation, the Evening Standard said.
The 16,250 square feet home is also not the largest in the road but it looks on to Kensington Palace. The 58-year-old Mittal, Britain's richest man, himself lives in a home in Kensington Palace Gardens which he bought for 57 million pounds four years ago and is three times-bigger than his latest acquisition.
The Mittals have carried out substantial improvements to their main home which is probably the largest private house in central London after Buckingham palace.
Mittal is believed to have offered 200 million pounds for a property owned by Foxtons founder Jon Hunt on the street.
The property, which is being enlarged underground, was the first choice of home for Mittal's son Aditya. However, Hunt turned down the offer.
The 32-year-old Aditya, chief finance officer of ArcelorMittal will be moving to Noam Gottesman's mansion. He, his wife Megha and their two daughters, wanted to be closer to his parents.
In March 2008, Mittal was named as the world's fourth wealthiest person by Forbes Magazine. His family owns 44 per cent of steel giant ArcelorMittal.