A portable electronic device includes a main body and a subsidiary casing. The main body includes a first side wall and a bottom wall. The first side wall defines at least one receptacle and at least one hole therein. The receptacle includes a cavity and a slot. The cavity spans from the bottom wall up into the main body. The slot is defined vertically in a part of the first side wall. The slot intercommunicates between the cavity and an exterior of the first side wall. The subsidiary casing includes a base and a cover. At least one latch and at least one connector are formed on the base. The latch is configured for being received in the receptacle and the connector is configured for being received in the hole.