In 1981, Galambos et al. found that a stimulus repitition rate of 40 per sec. caused the Middle Latency Response components to sum into a single, composite waveform of increased amplitude and named this response the 40Hz event related potential refered to the waveform's resemblance to several cycles of 40Hz sine wave. They considered that the relatively large amplitude of the 40Hz ERP and rapid rise in amplitude at very low sensation levels are characteristics which should facilitate the detection of the response, with consequent improvement in prediction of the auditory threshold. The authors have been used 40Hz ERP clinically from 1986. In these, We estimate of auditory threshold using 40Hz ERP on 28 normals, 56 ears, at frequencies of 0.5, 1, 2KHz and compared with PTA threshold at same frequency. The results were as following ; 1) Threshold difference between 40Hz ERP and PTA was obtained from 5dB to 40dB. Mean difference was 12.50dB at 0.5KHz, 14.82dB at 1KHz, 19.55dB at 2KHz. 2) Amplitude increased as stimulus frequency decreased. 3) Latency increased as stimulus frequency decreased. 4) With above results, we considered that the use of 40Hz ERP in the assessment of hearing threshold is desirable at low frequency.