A joining mechanism includes a main body and a positioning assembly. The positioning assembly includes a first positioning member, a second positioning member, a first resilient member and a second resilient member. The first and second positioning members are pivotally connected to the main body. When a first battery and a second battery are installed in the main body, the first and second batteries respectively push the first and second positioning members from a first initial position and a second initial position to a first release position and a second release position. When the first battery is removed, the second resilient member forces the second positioning member to rotate from the second release position to the second initial position and to join with the second battery.