Author has studied the pure tone delayed auditory feedback (DAF) to establish the standard as a objective hearing test with application of key tapping method. The subject was 38 ears of normal hearing and measured pure tone delayed auditory feedback effects at each frequencies, for evaluation of duration of test tone, delayed time and mode of DAF effect. The results were concluded as follows. 1) The most effective DAF effect was observed by application of 100msec of test tone and 200msec of delayed time. 2) The effect of DAF was getting better from lower frequency to higher frequency except 8000Hz. 3) The average threshold difference which was substracted standard pure tone threshold from DAF threshold measured by application of 100msec of test tone and 200msec of delayed time was 5.30±6.38dB at frequencies of 500-4,000Hz and 12.28±10.76dB at 125Hz and 250Hz. These two average threshold differences were reasonable as a standard for prediction of conventional pure tone threshold by DAF threshold. 4) The disturbed DAF threshold was measured by stimulation of synchronized tone to non-test ear with 20dB plus pure tone threshold and delayed tone to test ear with 100msec of test tone and 200msec of delayed time. The threshold difference of disturbed DAF which was calculated by substraction of pure tone threshold from disturbed DAF threshold at 500-4,000Hz was 26.24±7.79dB and then substraction of 20dB from threshold difference of disturbed DAF was 6.24dB. This 6.24dB was closely simiral to DAF threshold difference of 5.30dB. Therefore this disturbed DAF method also advocate reasonable method for objective hearing test. 5) The criteria of DAF effect were observed by disturbance of number and interval on key tapping, and incidence of these two distubances were closely related to each individuals but there is no specific tendency on frequency, delayed time and test tone. One of these effects appeared over 60% called tapping number dominant type and tapping interval dominant type and below 59% called mixed type. These types were observed 33.3% in each types.