Embodiments of a communication system, a base station (BS), a user equipment (UE), and a discovery method for device-to-device (D2D) communication are provided. The communication system includes a base station and multiple UEs. Each of the UEs uses one of multiple carrier frequency bands for communication. The BS obtains D2D communication requests of the UEs by the carrier frequency bands, and groups the UEs into multiple groups according to locations of the UEs. The BS allocates a discovery resource pool of a predefined carrier frequency band to the UEs in the same group and enables each of the UEs to transmit a discovery message corresponding to each of the UEs' own in the discovery resource pool.