The present invention provided a method and an apparatus for mapping radio resources in a wireless communication system. A terminal determines the size of an RBG (resource block group of a f basic band according to the number of RBs (resource blocks) within the basic band in the overall bands which include the fundamental band and an aggregated segment band. The size of an RBG of the aggregated segment band is determined on the basis of the size of the RBG of the basic band.