Objective: The purpose of this study was to compare the auditory P300 results of elderly patients with sensorineural hearing loss (SNHL) before and after hearing aid use. Material and Methods: A cross-sectional study was conducted from July, 2016 to April, 2017 at the Hearing Aid Clinic of Ramathibodi Hospital. Twenty-six elderly patients aged ≥60 with bilateral symmetrical SNHL who were referred for unilateral hearing aid fitting by otolaryngologist were included. The auditory P300 was recorded before and 2 months after hearing aid fitting. Results: The P300 waveforms, with a mean latency of 374.48 milliseconds and mean amplitude of 6.68 microvolts (µV), could be recorded in only 21 participants. At 2 months after hearing aid use, the mean P3 latency was 376.83 ms and mean amplitude was 8.77 µV. There was a statistically significant difference in amplitude of P300 2 months after hearing aid fitting (p-value=0.004). Conclusion: The auditory P300 results indicate an improvement in cognitive ability with higher amplitude. Thus, the P300 may be used to evaluate improvements in cognitive function after using a hearing aid. It can also be used as a guideline for explaining the benefits of hearing aid use to patients who initially rejected a hearing aid.