Disclosed are methods of performing handover and supporting thereof in a mobile communication system. The method of supporting handover at a first network entity in a mobile communication system comprises transmitting a plurality of data blocks of a first layer to a user equipment (UE), checking whether each of the plurality of data blocks is successfully transmitted to the user equipment (UE), and transmitting to a second network entity at least one data block of the first layer excluding a data block which is successfully transmitted to the user equipment (UE) among the plurality of data blocks of the first layer, the second network entity being a target network entity for handover of the user equipment (UE).