Proceeding of the Joint International Symposium on Deformatuon Monitoring. 2-4 November, 2011. Hong Kong. China. Report. Long time series from GPS have been the main tool in quantitatively analyzing crustal deformation. Many researchers have illustrated that the secular trend and periodic signals in GPS position time series could explain crustal deformation and its corresponding loading effects. Therefore, continuous GPS data collected from GPS Deformation Monitoring System can be used to obtain deformation properties of reservoir. In this paper we processed continuous GPS observations spanned 20 months from five GPS stations in Xilongchi Reservoir Deformation Monitoring System using DDMS software developed by us, obtained coordinate time series for 4-hour session solution. Then the long-term deformation of the reservoir was determined. Multipath effects were firstly investigated. It is shown that there were little difference between elevation cutoff angle of 15° and 25°. Results from the analysis of time series show that site L022 on the main dam moves north by east 81° at 1.9 mm/yr, while S071, north by east 109° at 2.2 mm/yr, L132, north by west 87° at 0.8 mm/yr. The rest 2 sites S171, S191 move westward slightly. Besides, all sites exhibit downward movement in various degree, among them L022 on main dam subsiding at the fastest rate of 3.8 mm/yr. Sites L132 (on auxiliary dam), S171, S191, and S071 subside at rates of 1.4 mm/yr, 0.5 mm/yr, 1.0 mm/yr and 0.8 mm/yr respectively. All sites, except for L132, show annual variations with an amplitude of ~ 1mm in east component, which suggests a correlation between annual signals and local temperature especially for sites L022 and TN01. Annual signals may be resulted from thermal expansion of the bedrock due to its seasonal characteristic.