A method of performing frame timing difference measurement in a wireless communication system, the method performed by a User Equipment, UE, and comprising: receiving measurement configuration information related to a measurement gap; in a state in which (i) an Evolved Universal Mobile Telecommunications System, UMTS, Terrestrial Radio Access, E-UTRA, cell is configured as a primary cell for the UE and (ii) a New Radio, NR, primary secondary cell, NR PSCell, is not configured for the UE : for frame timing difference measurement with a measurement gap: measuring a frame timing difference between the E-UTRA cell and an NR cell within a first detection time related to the frame timing difference, wherein the first detection time related to the frame timing difference is defined based on (i) a Synchronization Signal, SS/Physical Broadcast Channel, PBCH, Block, SSB, Measurement Time Configuration, SMTC, period and (ii) a repetition period of the measurement gap determined from the measurement configuration information; for frame timing difference measurement without the measurement gap: measuring the frame timing difference between the E-UTRA cell and the NR cell within a second detection time related to the frame timing difference, wherein the second detection time related to the frame timing difference is defined based on the SMTC period; and reporting, to the E-UTRA cell, a System Frame Number, SFN, and Frame Timing Difference, SFTD, based on measuring the frame timing difference. receiving measurement configuration information related to a measurement gap; in a state in which (i) an Evolved Universal Mobile Telecommunications System, UMTS, Terrestrial Radio Access, E-UTRA, cell is configured as a primary cell for the UE and (ii) a New Radio, NR, primary secondary cell, NR PSCell, is not configured for the UE : for frame timing difference measurement with a measurement gap: measuring a frame timing difference between the E-UTRA cell and an NR cell within a first detection time related to the frame timing difference, wherein the first detection time related to the frame timing difference is defined based on (i) a Synchronization Signal, SS/Physical Broadcast Channel, PBCH, Block, SSB, Measurement Time Configuration, SMTC, period and (ii) a repetition period of the measurement gap determined from the measurement configuration information; for frame timing difference measurement without the measurement gap: measuring the frame timing difference between the E-UTRA cell and the NR cell within a second detection time related to the frame timing difference, wherein the second detection time related to the frame timing difference is defined based on the SMTC period; and reporting, to the E-UTRA cell, a System Frame Number, SFN, and Frame Timing Difference, SFTD, based on measuring the frame timing difference. for frame timing difference measurement with a measurement gap: measuring a frame timing difference between the E-UTRA cell and an NR cell within a first detection time related to the frame timing difference, wherein the first detection time related to the frame timing difference is defined based on (i) a Synchronization Signal, SS/Physical Broadcast Channel, PBCH, Block, SSB, Measurement Time Configuration, SMTC, period and (ii) a repetition period of the measurement gap determined from the measurement configuration information; for frame timing difference measurement without the measurement gap: measuring the frame timing difference between the E-UTRA cell and the NR cell within a second detection time related to the frame timing difference, wherein the second detection time related to the frame timing difference is defined based on the SMTC period; and reporting, to the E-UTRA cell, a System Frame Number, SFN, and Frame Timing Difference, SFTD, based on measuring the frame timing difference. <EOS>