The DOJ is looking into "the competitive effects of the transaction," said Gina Talamona, a department spokeswoman.
Talamona would not comment on when the investigation began, or its scope.
The Wall Street Journal reported Wednesday that lawyers briefed on the case said the US Justice Department served Microsoft with a subpoena three weeks ago, requiring it to hand over documents relating to the investment.
The Justice Department is investigating whether Microsoft's stake in Corel will reduce competition in the market for word processing, spreadsheets and graphics software. Microsoft currently has 90 per cent of that market with its Office software package.
In October Microsoft invested $US135 million in loss-making Corel, best known for the WordPerfect word processing program.
Corel's decision to leave the market for the Linux operating system shortly after the investment by Microsoft has raised other antitrust concerns.
On 26 and 27 February, a U.S. Circuit Court appellate panel will hear Microsoft's appeal against the Justice Department's broader case. Microsoft lost that late last year when the U.S. District Court Judge Thomas Penfield Jackson ruled the company be split in two.
Talamona also would not comment on the DOJ investigation of Microsoft's $US1.1 billion pending takeover of business software maker Great Plains Software.