Abstract: The invention relates to a modular structure for heart beat signal wireless transmitter with fastening belts made of conductive and water fast fabric capably fixed on the underwear by sewing and washed together with the underwear, and with several different types of modular body capably replaced and installed on the fastening belt respectively to form the heart beat signal wireless transmitter; so that the heart beat signal wireless transmitter of this invention could make use of the fastening belts with conductive fabric of high flexibility as electrode to enable a close contact between the electrode and the user's skin to precisely detect the actual heart beat signal of the user.