A method of transmitting data in a wireless access system includes: calculating a number C of code blocks using a size B of an input bit sequence, a maximum size Z of the code blocks, and a size L of a cyclic redundancy check (CRC) which is to be attached to each of the code blocks; calculating a size B′ of a modified input bit sequence using the number C, the size L, and the size B; obtaining a size K of each of the code blocks using the size B′ and the number C; segmenting the input bit sequence to have the number C of the code blocks and the obtained size K of each of the code blocks; generating the code blocks by attaching the CRC to each of the segmented input bit sequences; and channel-coding the code blocks.