The present invention relates to a method for estimating a timing error and a frequency offset in a high-speed portable Internet system and a method thereof. A predetermined preamble symbol is added to an OFDM frame to be transmitted by a transmitter according to an exemplary embodiment of the present invention, an IFFT operation is performed, and then a pseudo noise is multiplexed by the preamble symbol to estimate the timing error and the frequency offset. A receiver receives an OFDM symbol, extracts the preamble symbol from the OFDM symbol, and multiplexes the extracted preamble symbol by a local pseudo noise, which is equal to the pseudo noise used by the transmitter but is shifted.