A data storage device and a data storing method thereof, including first main memories coupled to a plurality of channels, second main memories coupled to the plurality of channels in common, a buffer memory temporarily storing data to be programmed to the first and the second main memories; and a controller configured to program data of victim cache lines from the buffer memory to the second main memories while data of a first victim cache line from the buffer memory is being programmed to the first main memories. The storing method includes that a victim cache line is selected based on cost-based page replacement.