A FM transmitter and charger assembly includes a primary member, a FM transmitting device, and a securing member; audio signals in a MP3 player can be transferred to, and transmitted by the transmitting device when both the MP3 player and the FM transmitting device are connected to the primary member while a radio can receive the signals, and play music accordingly when it is tuned to the same channel as the transmitting device; the securing member includes a hollow part for the MP3 player to be closely inserted therein, and an engaging plate connected to a back of the hollow part and capable of being detachably fitted onto the primary member; thus, a certain primary member allows different MP3 players to be securely connected thereto with the help of various securing members, which have different sizes for suiting MP3 players with different sizes.