An effective SCELL activation procedure is proposed to reduce the SCELL activation time. For carrier aggregation, a secondary cell (SCELL) needs to be configured and then activated before a UE can perform normal operation. In a first embodiment, the UE performs measurements on a list of candidate SCELLs before SCELL configuration. In a second embodiment, the UE performs measurements on a configured SCELL with high priority after the SCELL is configured. In a third embodiment, the UE performs deactivated-state SCELL measurements with adaptive SCELL measurement interval based on a configured measurement cycle and a state machine. Small SCELL activation time allows energy saving and network scheduling gain.