Do you mean a ceiling fan? They are not the most efficient way of ventilating the coop if that's what you're after, although they will stir the air inside the coop which may be somewhat beneificial in a few circumstances.
If you use a ceiling fan (or really any other fan) you ought to make sure to spend extra to get one designed for agricultural or workshop use, because coops are dusty and you can a) seriously shorten a fan's lifespan if it isn't designed for that, and more serously b) set things on fire if dust gets into the wrong part of the works.
Pat, with ceiling fans in the building the chickens are in because it started as someone's dog boarding kennel with ceiling fans to keep dogs cool in summer... but if I ever needed active ventilation I would get an agricultural through-the-wall fan with a hood.