The Taipei basin is a triangle-shaped formation. The ground surface of the Basin is almost flat, but tilting gently to the northwest. The geological structure inside the basin consists of Quaternary layers above the Tertiary base rock. A dense strong motion observation network has operated in the Taipei area since 1991, as part of the Taiwan Strong Motion Instrumentation Program (TSMIP). Characteristics of site effects in the Taipei basin are studied using the data recorded by this observation network. In this study, the method of amplitude spectrum ratio is used to analyze site effects in the Taipei basin. This analysis clearly shows that the spectral ratio contours at low frequency bands (0.2 ¡V 1 Hz) correlate with the structures of the Tertiary basement shape and the top soft soil layer. For the higher frequency band (1 ¡V 3 Hz), the main amplification effects occur near the north, east, and south basin edges. This shows that the edge effect found in the Kobe earthquake could possibly occur in this area. Dominant frequencies in the Taipei basin area are also given in this study. This will benefit the microzonation work for future earthquake hazard mitigation in the Taipei basin.