A key button mechanism and a portable electronic device using same are provided. The key button mechanism includes a base body, a protective piece and a key button. The protective piece is assembled to the base body and includes two actuating portions. The key button is partially attached to the base body and aligns with the protective piece. The key button includes a key body and two resisting blocks formed on the key body. The two resisting blocks are configured to align with and abut against the corresponding two actuating portions of the protective piece.