A base station and a device to device (D2D) user equipment (UE), a transmission method, a report method and a resource adjustment method for a wireless communication system are provided. When the D2D UE operates as a transmitting end, it is able to determine a transmission mode according to a transmission condition (e.g., whether a radio link failure or a collision occurs when a D2D transmission is being performed, or whether the telecommunication operators of the transmitting end and the corresponding receiving end are the same). When the D2D UE operates as a receiving end, it is able to transmit a report message carrying received signal quality to the base station after receiving D2D data. Accordingly, the base station is able to adjust a D2D resource configuration according to the report message.