The company says smoke was found in a room inside a protected area of the plant around 9 a.m. The smoke stopped once electricity was cut to the electrical equipment inside the area.
Exelon says no injuries have been reported, and there was no release of radiation.
The company also says the incident didn't affect operations at the plant, which is located about 25 miles southwest of Rockford.
A plant spokesman says no one at the plant saw any flames, just smoke.
The company classified the fire as an unusual event, which is the lowest of four emergency classifications defined by the Nuclear Regulatory Commission.