A method for transmitting data in a mobile communication system. The method comprising segmenting a data stream into at least one consecutive frame having a variable data length, the data stream being segmented into a plurality of consecutive blocks having a variable data length, each said consecutive block being segmented into a plurality of sub-consecutive blocks having a byte length; attaching, at each head of the consecutive frames, a header including a first set of bits indicating the sequence number of a consecutive block corresponding to the head and a second set of bits indicating the sequence number of a sub-consecutive block corresponding to the head; and transmitting the header-attached consecutive frames.