An efficient method on selecting threshold values according to minimax test for packet detection in burst-mode OFDM systems is proposed. Packet detection decides whether a packet is coming or not by comparing a threshold value in the wireless receiver. Related with sliding window size and SNR, the threshold value affects receiving performance including probabilities of false alarm and miss. The minimax test for detection based on empirical CDF and survival functions is proposed. Also the performances of two general used detection methods are surveyed and compared.