A heart rate monitor assembly includes a data processing unit which includes a case in which a data processor and a power supply device are received. The case includes two L-shaped holes and two conductive plates are received in the case and located in the two L-shaped holes and connected to the data processor. A belt has two connection ends and each connection end has an L-shaped connector which is removably engaged with the L-shaped hole corresponding thereto and connected to the two conductive plate.