The present invention relates to a method of transmitting a signal by a user equipment (UE) in a wireless communication system. Especially, the method includes the steps of receiving a handover command containing configurations for a second network, from a first network; performing a random access procedure to the second network; re-establishing a packet data convergence protocol (PDCP) entity associated with a signaling radio bearer (SRB) based on the configurations for the second network, after a random access procedure to the second network successes; transmitting a radio resource control (RRC) message to the second network using the re-established PDCP entity.