Abstract: In this paper, we introduce three methods for determining dominant period based on the strong motion records, they are ① the Fourier spectral analysis method of surface records； ② horizontal/vertical Fourier spectral ratio method； ③ surface/underground Fourier spectral ratio method. Several hundred strong motion records from two bedrock stations and two stations on site class Ⅲ in the KiK-net strong motion observation network in Japan are used to analyze the site dominant period. It is concluded that， on the bedrock site， the dominant frequency obtained from the three methods are scattered due to the hard rock site condition which has little effect on the ground motion. On the soil site， the overlying soil has great effect on the ground motion， and accurate values of dominant period can be obtained from the three methods. The dominant frequency from the surface/underground Fourier spectral ratio method is more accurate than that from the surface horizontal/vertical Fourier spectral ratio method whose result is uncertain on some soil sites.