Abstract: A plastic ceiling box assembly adapted to support a ceiling fan thereon and adapted to be mounted on a structural member, such as a ceiling joist. The ceiling box comprises a body member having a lower wall and a pair of side walls defining a recess for snugly receiving the ceiling joist therein. Box mounting holes for receiving box mounting fasteners are located at opposite sides and ends of the lower wall for attaching the ceiling box to the joist. Openings for receiving fan supporting fasteners are formed in the body member adjacent the box mounting holes. Brackets at opposite ends of the body member aid in attaching the body member to the joist. This overall arrangement provides sufficient support and strength to resist dynamic loads of a ceiling fan weighing up to 35 pounds even though the ceiling box is formed of plastic. The plastic ceiling box assembly could be provided with a substantially U-shaped metal bracket for supporting ceiling fans weighing over 35 pounds.