An apparatus and a method for determining a Channel Quality Indicator (CQI) using a beamforming in a multi-antenna system are provided. A transmitter for determining a CQI based at least partly upon the beamforming in the multi-antenna system includes a beam former for distinguishing frequency tone intervals having a constant channel across an entire frequency tone, forming a beam by multiplying frequency tones of the frequency tone interval having the constant channel by different beamforming weights, and transmitting a preamble signal. Hence, the users can be scheduled based at least partly upon the beamforming gain, and throughput can be enhanced through the practical MCS selection.