A battery cover assembly for a portable electronic device (100), the battery cover assembly includes a cover (10), a housing (30) and a button mechanism (20). The cover defines a button hole (12). The housing defines a first stopper groove (345) and a second stopper groove (365). The button mechanism includes a slidable element (22), a latching element (24) and a swinging element (26). The slidable element includes an operation portion (2224) and a fixed portion (226). The operation portion is slidably received in the button hole, and the fixed portion is releasably locked in the first stopper groove. The latching element includes an extending portion (244) releasably locked in the second stopper groove. The swinging element connects the slidable element to the latching element.