A method of generating a data block in a wireless communication system is provided. The method includes preparing a first upper data block, a second upper data block and a third upper data block and generating a lower data block comprising a header, the first upper data block, the second upper data block and the third upper data block, the header comprising a first length indicator and a second length indicator, wherein the first length indicator indicates the length of the first upper data block and the second length indicator indicates the length of the second upper data block. Data rate can be improved by reducing the size of a header of data blocks.