Gina Talamona, a DoJ spokeswoman, said the department is looking into "the competitive effects of the transaction". 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 DoJ served Microsoft with a subpoena three weeks ago, requiring it to hand over documents relating to the investment.
Word-processing competition The DoJ is investigating whether Microsoft's stake in Corel will reduce competition in the word-processing, spreadsheets and graphics markets. Microsoft currently has 90 per cent of that market with its Office software package.
In October, Microsoft invested $135 million in loss-making Corel, which makes Word Perfect word-processing program.
OS concerns Corel's decision to leave the market for the Linux operating-system shortly after the investment by Microsoft has raised other antitrust concerns.
On Feb 26 and 27, a US Circuit Court appellate panel will hear Microsoft's appeal against the DoJ's broader case. Microsoft lost that late last year when US 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 $1.1 billion take-over of business-software maker Great Plains Software, which is still pending.