Purpose: A number of hearing loss had gradually been increased due to several reasons including newborn hearing screening, social environmental, and sociodemographic factors. In most cases, hearing loss is treated with the use of hearing aids. Hearing aids are composed of several parts, and the mold is a primarily component. Horn, damper, and vent of mold can control the sound pressure level in the ear canal, and furthermore, the vent can reduce the occlusion effect by controlling the low frequency of below 1,000 Hz. This study aimed to acquire basic information for audiological rehabilitation by investigating the effect of a vent diameter on the external auditory canal. Methods: Thirty-three ears of 18 subjects were participated. They had no problem with external and middle ears. Real ear measurement was used. Real ear unaided gain (REUG) and real ear aided gain (REAG), which were found by opening vent diameter 0, 1.1, 2.1, 3, 4 mm, and open ear canal. The study compared the differences between REUG and REAG, statistically. Results: There was observed significant gain differences at below 400 Hz in diameter 1.1, 2.1 mm, below 500 Hz in diameter 3, 4 mm, and below 600 Hz when the ear canal opens. Conclusion: These findings will be expected to be useful information to predict the vent effect in audiological rehabilitation using hearing aids.