The present disclosure relates to a method and apparatus for determining a Transport Block Size (TBS) in a wireless communication system, and more particularly, to a method and apparatus for determining a TBS table in association with 256 QAM. Particularly, a method for a base station to transmit data includes receiving Channel State Information (CSI) from a User Equipment, determining a Transport Block Size (TBS) value based on a TBS table including indices corresponding to the 256QAM modulation scheme and the number of allocable Physical Resource Block (PRB) pairs, and transmitting data using the TBS value.