Abstract: A chain saw sharpener is provided for sharpening the teeth of a cutting chain during operation of the chain saw. The sharpening system includes a housing mounted in the chain saw engine shroud and projecting both interior of and exterior of that shroud. A bushing is splined within the housing and the arbor of a grinding stone is rotatably supported in the bushing. The bushing and grinding stone are movable with respect to the fixed housing between a first position in which the grinding stone is remote from the cutting chain teeth and a second position in which the grinding stone is positioned for contact with cutting edges of the chain teeth as they travel in their fixed path around a sprocket within the shroud. A manually depressable knob is threadably engaged with the bushing and includes a stop surface engagable with the housing to define the second position of the grinding stone.