A mobile communication system capable of increasing communication efficiency and a method therefor are provided, including a receiver for grouping n MCS levels (where n>0) into continuous m MCS levels (where n>m>0) according to the quality of a data channel for transmitting an information signal, and sending information on an MCS group using a pilot signal, and an ACK or an NACK signal, which indicates whether the information signal is completely received; and a transmitter for storing information on the n MCS levels, extracting the m MCS levels according to the information on the MCS group transmitted from the receiver, determining one of the extracted m MCS levels as an initial MCS level, modulating and coding information data to be transmitted according to the initial MCS level and transmitting the result to the receiver, and changing the current MCS level in response to the ACK or the NACK signal.