Abstract: Net primary productivity (NPP) is the foundation of ecosystem services. Based on the Landsat TM/ETM+imageries and daily meteorological data, the land use and land cover change (LUCC) of Jiangyin County were studied, and the NPP of terrestrial ecosystems was calculated with a process-based model, the economic value of CO2 assimilation was assessed by the methods of afforestation cost and carbon tax meanwhile. The results showed that the total mean NPP decreased from 818g/(m2?a) to 699g/(m2 ? a) and the economic value of carbon assimilation dropped by 88 million yuan (RMB) from 1991to 2002. The sprawl of residential area and the reduction of forest area and croplands during this period were the most important reason for the decline of mean annual NPP, the degradation of vegetation quality served as second factor, while the influence of climate changes was not noticeable.