In this paper, we propose a preamble detection scheme in the frequency domain that has not been attempted until now for OFDM based WLAN systems. We explain the reason for trying the preamble detection method in the frequency domain and analyze the reason why the preamble detection method has never been tried in the frequency domain. By introducing the concept of sample-based FFT, we propose a solution to enable sample-based operation even in the frequency domain. The proposed algorithm shows improved performance in terms of SNR than the existing algorithm.