There is no specific method for treatment of noise induced hearing loss, therefore we can say prevention is the best way. The purpose of this study is to set up the preventive indices for noise-induced hearing loss by investigating discrimination scores of mono-syllable words after noise stimulation. The subjects were 87 persons(43 male, 44 female) whose average hearing level was less than 15dB, especially less than 10dB at 4,000Hz and their speech reception threshold(SRT) was less than 25dB, the discrimination score was 100% in 40dB sensation level(SL) of SRT. The stimulated sounds were narrow band noise of 4,000Hz and white noise. The stimulated intensity was 90dB above the pure tone threshold in each subject. First, noise stimulation was given for 10 minutes, then a speech discrimination score in 40dB SL of SRT and its recovery time was checked immediately after noise stimulation. The results was as follows : 1) Speech discrimination score in 40dB SL was 87.3% after white noise stimulation and 85.6% after narrow band noise. There was statistical difference between white and narrow band noise(p<0.05). 2) There was no statistical difference according to age and sex in speech discrimination score and recovery time after noise stimulation. 3) Subjects who had a speech discrimination score below 76% after white noise stimulation, below 74% after band noise stimulation, and their recovery time over 6 minutes were at risk of developing noise induced hearing loss.