1. A transmitting apparatus for transmitting a data stream in a multiple antenna system, the transmitting apparatus comprising: a plurality of quantization codebooks corresponding to a number of data streams and formed by quantizing channel information between a transmitting antenna and a receiving antenna; a modulator for determining the number of data streams to be transmitted based on an index of a selected stream mode in feedback information transmitted from a receiving apparatus wherein the stream mode is selected at the receiving apparatus and is defined to determine a modulation method applied to each data stream at the transmitting apparatus, and modulating at least one data stream corresponding to the number of data streams based on the index of the selected stream mode; and a quantization precoder for precoding the modulated data stream based on a codeword index in the feedback information transmitted from the receiving apparatus among the quantization codebook corresponding to the stream mode of the index, and transmitting the precoded data stream to the receiving apparatus.