A mode selection device and a multi-function button containing such a device are provided. The mode selection device includes a case, a chassis and a dial. The case has a hole; the chassis, lying under the case, includes a latch and a set of slots. The latch is between the slots. The dial, installed in the hole, includes a recess and a set of protuberances corresponding to the slots. The recess is located in the center of the dial. The protuberances are located under the dial and engage with the slots. When the latch engages with an edge of the dial, the latch, the slots and the protuberances together keep the relative position between the dial and the chassis unchanged.