Sustainable development of geothermal resources is the key to prolong the commercial life of geothermal fields. This paper uses two well-known geothermal fields in China, with long histories of production, to demonstrate how sustainability can be quantitatively evaluated, and to highlight the essential differences between sustainable and non-sustainable development schemes. After examining the complex nature of sustainability evaluation, fuzzy synthetic evaluation is applied as a tool for the quantitative sustainability rating of geothermal reservoirs. The evaluation procedures include systemic criteria selection, weighing, individual criterion evaluation and finally final multi-criterion decision analysis evaluation. The sustainability of a limestone reservoir in the urban sector of Tianjin is rated rather low because of insufficient levels of water injection. On the contrary, the carbonate rock reservoirs of the Xiaotangshan geothermal field in suburban Beijing have a strong sustainability rating, mainly due to high injection rates. (C) 2011 Elsevier Ltd. All rights reserved.