The absolute and the relative bone conduction threshold mean the level of loudness respectively with the occlusion and the opening of external ear canal. The difference between the absolute and relative bone conduction threshold is called the occlusion effect. In order to study the audiologic significance of occlusion effect and occlusion index which correlated to the occlusion degree, air-bone gap, recruitment phenomenon and static compliance in such frequency as 250Hz, 500Hz, 1000Hz, 2000Hz and 4000Hz, subjects of 184 normal ears, 123 ears of conductive hearing loss and 91 ears of sensorineural hearing impairment were analyzed. The following results were obtained. 1) Absolute bone conduction threshold occluded with earphone was 8.08±4.18dB at 250Hz, 6.54±4.11dB at 500Hz, 5.00±3.92dB at 1000Hz, 0.38±2.34dB at 2000Hz and -0.91±1.93dB at 4000Hz in normal group. These data indicated the occlusion effect appeared at 250Hz, 500Hz and 1000Hz but negligible at 2000Hz and 4000Hz in normal group. 2) The occlusion index was 29.30±12.40dB in case of sensorineural hearing loss. 3) The occlusion effect changed in accordance with occlusion methods and the occlusion effect by the earphone was less than that by cotton ball. 4) The occlusion index was not related with degree of PTA threshold in conductive hearing loss group but sensorineural hearing loss group bigger in proportion to PTA threshold. 5) The occlusion index had no relationship with air-bone gap in conductive hearing loss group. 6) There is no difference of occlusion index between the SISI positive group and SISI negative group. 7) The static compliance in normal group was 0.93±0.43cc and the occlusion index became smaller in inverse proportion to the static compliance. The auther concluded that no difference between the absolute and the relative bone conduction threshold suggests a lesion of the conductive mechanism and if the relative bone conduction threshold is higher than the absolute bone conduction threshold, it is an indication that the cause of hearing loss is due to a sensorineural origin.