A portable electronic device with a fastening mechanism includes a main body and a subsidiary casing. The main body comprises a bottom wall and a side wall extending from one edge of the bottom wall. A receptacle is defined in the sidewall thereof. The subsidiary casing is configured for receiving a battery therein. The subsidiary casing comprises a latch formed thereon. The latch is engaged into the receptacle to prevent the subsidiary casing from moving along a first direction, wherein the latch is inserted into the receptacle along a second direction that is perpendicular to the first direction when the subsidiary casing is assembled to the main body.