A method and device are disclosed from the perspective of a first Wireless Transmit/Receive Unit (WTRU). In one embodiment, the method includes the first WTRU triggering a first buffer status report (BSR), wherein the first BSR indicates an amount of data expected to arrive. The method also includes the first WTRU triggering a second BSR, wherein the second BSR indicates an amount of data already available. The method further includes the first WTRU generating a MAC (Medium Access Control) PDU (Protocol Data Unit), wherein the MAC PDU includes at most one first BSR MAC CE (Control Element) for the first BSR and at most one second BSR MAC CE for the second BSR.