The present invention relates to a wireless communication system. More specifically, the present invention relates to a method and a device for handling for an uplink split operation in wireless communication system, the method comprising: generating and/or receiving second data available for transmission in a PDCP entity, and determining a RLC entity to be used for submitting the second data based on first data volume, if second data volume is larger than a first threshold and less than a second threshold; indicating the second data volume to a first MAC entity associated with a primary RLC entity, if the first data volume is less than the first threshold; and indicating the second data volume to both the first MAC entity associated with the primary RLC entity and a second MAC entity associated with a secondary RLC entity, if the first data volume is larger than the second threshold.