A method of supporting active bandwidth part (BWP) switching under carrier aggregation (CA) is proposed. To avoid longer switching delay and multiple interruptions in other component carriers (CCs)/cells, the starting time of the later active BWP switching in one cell should fall outside the switching delay of the earlier active BWP switching in another cell. If the later active BWP switching is DCI-based, then the network should schedule the later active BWP switching outside the switching delay of the earlier active BWP switching. If the later active BWP switching is timer-based, then the UE should not perform the later active BWP switching until the earlier active BWP switching is completed.