A bottom simulating reflection (BSR) that parallels the sea floor occurs on the seismic profiles from Hydrate Ridge, Oregon continental margin. In 2000 a 4 X 10 km (super 2) 3-D high-resolution seismic survey was conducted on south Hydrate Ridge on the Oregon continental margin. The objective of the survey is to characterize the regional pattern of fluid and gas migration and its relationship to hydrate accumulations on Hydrate Ridge. These data were acquired with a high-resolution seismic source with source-receiver offsets of up to 644 m, which results in incidence angles of up to 20 degrees at BSR. In 2002 new 2D seismic data with source-receiver offsets of up to 1500 m, producing incidence angles of up to 40 degrees at BSR, were collected in Ewing Cruise EW0208 in coordination with Ocean Drilling Program (ODP) Leg 204. My goal in this thesis is to use 3-D high-resolution seismic data offshore Oregon to evaluate the distribution of free gas and gas hydrate by amplitude-versus-offset (AVO) analysis, and to relate the distribution of free gas and gas hydrate to the structure and stratigraphy of Hydrate Ridge. Prior to AVO analysis, I conducted the true amplitude recovery by using the seismic range equation, and the true amplitude processing through prestack time migration. Although the 3-D high-resolution seismic data is of good S/N ratios, I found by observation and computation that the amplitude in the 3-D data is anomalous or incorrect. So I also calibrated the 3D data by using the new 2D data to remove unfavorable acquisition effects in the 3D survey. On prestack-migrated gathers, I picked up the seismic amplitude from the three surfaces associated with free gas accumulations: 1) the bottom simulating reflection (BSR), 2) Horizon A, and 3) Horizon B'. Intercept and slope are obtained from AVO fitting, and the Poisson's ratio obtained by using intercept and slope with the constraints of well-log data. Free gas saturation for the three surfaces is estimated by fluid substitution technique. The estimated gas saturation from seismic data is compared with that from the resistivity and the pressure core sampler (PCS)