Groundwater fluctuation in the coastal region is affected primarily by tidal oscillation. This study aimed to identify the effects of ocean tides on groundwater levels in the coastal aquifer of the Taoyuan-Hukou Table plain, Taiwan. In this study, Fast Fourier Transform (FFT) analysis was conducted to investigate the frequency of groundwater level based on the field monitoring data. The field observation showed that groundwater level fluctuates following the tide and fluctuations similar to a waveform, which suggested tidal wave-induced groundwater level changes periodically. The frequency analysis indicated that groundwater fluctuation was influenced by the multi constituents tide and significant component of the semidiurnal tide (M2) dominant with a period of 12.3 hours. The results also indicated that the fluctuation amplitudes decreased exponentially, and phase lag increased linearly for dominant tidal signals as they propagated inland. The tidal effects on the groundwater level reached up to 400 m inland from the coast.