The present invention relates to a method of transmitting data in a handover between base stations. When a mobile terminal performs a handover between base stations in a wireless communication system, a gateway transmits data traffic to be transmitted to a mobile terminal by using an IGMP (Internet Group Management Protocol) to a serving base station being serviced to the mobile terminal and a target base station to which a handover of the mobile terminal is made in a bi-casting method. Accordingly, when the mobile terminal performs the handover between the base stations, the gateway transmits the data traffic in the bi-casting method. As a result, it is possible to efficiently use a resource of a user plane connection interval between the gateway and the base station, thereby minimizing the loss of data traffic transmitted in a downlink, that is, data.