In order to study the characteristics of resonance frequency depending on different shapes of the external ear, authors measured resonance frequencies and their sound pressure levels using 6,500 Hearing Aid Test System in adults(32 ears), children(28 ears) and patients who underwent open cavity mastoidectomy(17 ears). The following results were obtained : 1) In adults, mean resonance frequencies of the external ear, auricle and external auditory canal were 3,655Hz, 6,445Hz and 3,464Hz respectively and their sound pressure gains were 18.9dB SPL, 15.2dB SPL and 12.6dB SPL, repectively. 2) Children showed higher resonance frequency of the auricle(6,931Hz) than that of adults, and resonance frequencies of the external ear(3,351Hz) and external auditory canal(3,701Hz) were similar to those of adults. In addition, sound pressure gains of the external ear(20.1dB SPL) and external auditory canal(15.4dB SPL) were higher than those of adults. 3) Patients underwent the open cavity mastoidectomy had lower resonance frequencies of the external ear(1,960Hz), auricle(4,213Hz) and external auditory canal(2,430Hz) than those of normal adults. However, sound pressure gains of the external ear(22.6dB SPL) and external auditory canal(20.8dB SPL) in patients were higher than those of normal adults. Above results indicated that different shapes of the external ear may affect the resonance frequency and the accurate sound pressure measurement will be needed around the resonance frequency range for the hearing aid selection.