Quantitatively setting ecological compensation criterion is a focus and a difficulty in establishing ecological compensation mechanism. Based on the 1987-2007 meteorological data and the 2007 social survey data of Gannan Tibetan Autonomous Region, the supply curves of grassland ecosystem services in the Region were simulated by the methods of minimum-data, social survey, and precipitation storage estimation. The opportunity cost of implementing ecological compensation in the Region in 2007 was 503.6 yuan x hm(-2), per unit water conservation volume supplied by grassland ecosystem was 1110.7 m3 x hm(-2), and ecological compensation criterion was 788.6 yuan x hm(-2). The total water conservation volume supplied by grassland ecosystem was 2.56 x 10(9) m3.