A hinge assembly, including a rotating shaft and a pair of cams, is provided. The cams are sleeved on the rotating shaft so as to be rotatable relative to each other. The pair of cams respectively has a first flat surface and a second flat surface facing each other, and a curved surface adjacent to the first flat surface and the second flat surface. The first flat surface of one of the cams passes by the curved surface to move from one flat surface to the other flat surface of the other cam, so as to enable the pair of cams to move relatively away or closer along an axial direction of the rotating shaft. The curved surfaces of the cams are in surface contact when the first flat surface passes by the curved surface. A portable electronic device is also provided.