One vertical component and two horizontal components of 27 oceanbottom seismometer (OBS) data sets along four profiles off eastern Taiwan were obtained using a large air-gun array of the R/V Maurice Exing during summer/fall 1995. From the horizontal compoents of the OBS data, we identify most refractions converted in the sediment and reflections converted in the crust respectively as the first and later arrivals. These converted arrivals are subsequently used for travel-time modeling of the sedimentary and crustal Poisson's rations. The overall results show differences of Poisson's ratios in the accretionary prism (0.28~0.3), the continental crust (0.25~0.29) and the oceanic slab (0.23~0.33), and these are associated with the different structural compositions and/or tectonic settings. OBS data modeling of profile EW9509-1, which covers the Ryukyu arc-trench system in the north-south direction, reveals relatively low Poisson's ratios in the northern portion of the Yaeyama accretionary prism (0.28) and in the southern portion of the Ryukyu Arc basement (0.25). Anomalous Poisson's ratios within the accretionary prism and the continental crust may be attributed to the fractures of the forearc compression beneath the Nanao Basin. From the east-west profiles of EW9509-14 and EW9509-16 across three forearc basins, we also find anomalously high Poisson's ratios (0.27~0.29) for the Ryukyu Arc basement beneath the Nanao Basin. High Poisson's ratios of the overlying plate suggest the presence of fluid-filled micro-cracks, which may be related to the northward movement of the Luzon Arc and the Gauga Ridge, Poisson's ratio decreases northwestward along profile EW9509-23 that is consistent with the increasing crustal thickness northwestward in the P-wave modeling below the Huatung Basin.