A method of assigning radio resources in a cellular system using wired Relay Stations (RSs) includes: a Base Station (BS) gathering feed-back control information of all Mobile Stations (MSs) via the RSs in a cell of the BS; a first process, wherein the MSs are divided into two groups according to a channel state of each MS; a second process, wherein sub-channels are dynamically assigned to MSs belonging to a group, in which a channel state of each MS is good, of the two groups; and a third process, wherein sub-channels are dynamically assigned to MSs belonging to a group, in which a channel state of each MS is bad, of the two groups by considering a resource state per RS and a channel gain between sub-channels.