A base station (BS) for handling buffer status reporting comprises a storage device and a processing circuit coupled to the storage device. The storage device stores, and the processing circuit is configured to execute instructions of: configuring a radio bearer (RB) and configuring a first logical channel (LC) and a second LC associated to the RB, to a communication device; configuring the communication device to report separate buffer statuses for the first LC and the second LC associated to the RB; receiving a first buffer status comprising a volume of data associated to the first LC and a second buffer status comprising a volume of data associated to the second LC, from the communication device; transmitting a first UL grant to the communication device according to the first buffer status; and transmitting a second UL grant to the communication device according to the second buffer status.