The present invention relates to a wireless communication system. More specifically, the present invention relates to a method and a device for calculating an amount of data available for transmission in the wireless communication system, the method comprising: receiving ratio for calculating amount of Data Available for Transmission (DAT) in a PDCP (Packet Data Convergence Protocol) entity; calculating an amount of DAT when data is arrived in the PDCP entity; and setting a first amount of DAT as the calculated amount of DAT and a second amount of DAT as ‘zero’, if the calculated amount of DAT is less than a threshold, wherein the first amount of DAT is for the first BS and the second amount of DAT is for the second BS.