A method and apparatus for performing Reed-Solomon (R-S) coding in a mobile communication system supporting a broadcast and multicast (BCMC) service are provided, where a determination is made as to whether a change in a broadcast overhead message (BOM) exist including information with parameters used for the BCMC service. A position of data in a new channel encoder buffer is calculated at a time when the BOM is changed. The channel encoder buffer is filled with predetermined data in an interval ranging from a start point of a buffer comprising data in the change point of the BOM to a point preceding a point of data first transmitted according to the changed BOM.