Because Nokia is a European-based firm, antitrust regulators will critique the takeover bid to ensure the competition is not left at a disadvantage.
Authorities can block the deal, or fine the companies heavily.
According to Reuters, the Commission said it would decide by December 4 whether to clear the acquisition. It can extend the review by a further 10 business days if Microsoft offers concessions to allay concerns.
Over in India, authorities have already approved the deal, which kicks out another regulatory hurdle out the way. According to Indian media, authorities said the deal is "not likely to have appreciable adverse effect on competition."
The bid for Microsoft to acquire Nokia will see the Redmond, Wash.-based software giant become a fully-fledged phone maker, years after it evolved its Windows Mobile platform into Windows Phone.
Microsoft will absorb the company's smart devices unit, which develops the Lumia array of smartphones, allowing Microsoft to directly control both the hardware and software ecosystem.
The deal is expected to complete around the first quarter of 2014, and will also be subject to U.S. regulators.
Meanwhile, Nokia will hold an extraordinary shareholders meeting on November 19 to approve the sale of the company's devices and services to Microsoft.