A shelf includes a rear frame, two side frames pivotally mounted on two opposite ends of the rear frame respectively, two connecting frames each detachably mounted on the rear frame and each located between the two side frames to locate the two side frames, and at least one placement board mounted between and supported by the rear frame and the two side frames. Thus, the shelf can be folded to have a smaller volume so as to decrease the cost of packaging, storage and transportation of the shelf.