Bone-conduction threshold is essential for differentiation of the type of deafness and decision of prognosis. There are many factors influencing the bone-conduction threshold testing and bone-conduction vibrator placement is also an important factor for obtaining more accurate threshold. For evaluation of more accurate placement, author studied bone-conduction thresholds forehead and mastoid portions with 100 ears of normal listeners in the last year. Now studied bone-conduction threshold of conductive deafness 50 ears and sensorineural deafness 43 ears with the forehead and mastoid bone vibrator placement using the Nagashima 51A-T72 audiometer(ref.=ISO). Author conclude the result as follows. 1) In conductive deafness, bone-conduction threshold of the forehead was 2.3~11.7dB, -3.0~5.5dB at mastoid tip and -3.1~5.9dB at mastoid upper portion. There was a tendency the higher of frequencies, the more of thresholds and gradual decrease of threshold at high frequency. 2) In conductive deafness, test-retest difference were 2.4~3.2dB at the forehead, 2.5~3.7dB at mastoid tip and 1.8~3.2dB at mastoid upper portion, and there was no significant difference at all frequencies in three locations. 3) In conductive deafness, the difference between the mean thresholds of the mastoid tip and mastoid upper portion was meaningless but the difference between the forehead and mastoid portions was about 4.4~7.3dB. The most average difference was at 500Hz(7.2~7.3dB) and the least was 2K~4KHz(4.4~4.9dB). 4) In sensorineural deafness, the forehead bone conduction threshold was 23.6~49.5dB, 14.8~48.0dB at mastoid tip and 14.4~47.3dB at mastoid upper portion. There was also an increasing tendency of the threshold from lower frequency to middle one and decreasing to high frequency again. 5) In sensorineural deafness, test-retest difference of the forehead was 2.6~5.7dB, 3.7~5dB at mastoid tip and 2.2~4.1dB at mastoid upper portion, and there was no significant difference between the frequencies and locations. 6) In sensorineural deafness, the difference of mean thresholds between mastoid portions was meaningless except 500Hz as 4.6dB but the difference between the forehead and mastoid was about 1.4~12.9dB. The largest average difference was at 250~500Hz(8.2~12.9dB) and the smallest was 2K~4KHz(1.4~2.8dB). 7) Author recommand the mastoid measurement than forehead in bone-conduction testing because we found that the former is easier in masking and its threshold is more accurate than the later at normal listener and deafness cases.