Stena Bulk has ordered two Suezmax tankers from Samsung, South Korea for delivery in 2011.
The company said the 158,700dwt vessels, Stena Superior and Stena Supreme, measuring 274m in length, will be the largest tankers in its fleet.
The US tanker shipping company is also planning to sign a contract for two optional sister vessels, which will be announced in spring this year.
The company has invested nearly $7m per vessel for environmental modifications.
The vessels will be included in the high-class tonnage of the Stena Sonangol Suezmax pool, which consists of around 15 tankers and will be expanded to about 25 large tankers over three years, the company said.