Abstract: Since 1980s，more and more attention has been paid to the study on ecosystem function. The estimation of ecological value has become a new hotspot in ecology development. Returning farmland to forest & mountain  enclosure is a major forestry ecological project in China. What are the ecological effects of the project，and is it economically reasonable? This article explored the relative ecological functions and estimated the relative ecological value，taking the sub-project of returning farmland to forest & mountain enclosure in Yongxing Township of Jingyuan County  as a case study. The results showed that：（1） The ecological value of the forest  is 13.09~25.09 times of that of crops ，which is far more than the sum of the ecological value of farm crops and the agricultural output，and also more than the national subsidy standard. So the state policy of returning farmland to forests is not only favorable to local eco-environment restoration，but also reasonable in economic development. （2） The ecological value of mountain enclosure is less than the opportunity cost of prohibiting grazing，also less than the national subsidy standard. The sum of the relative ecological value of prohibiting grazing and the state subsidies is more than the opportunity cost of prohibiting grazing. However，the ecological value of new vegetation through returning farmland to forest project is greater than China's average ecological value of grassland. The relative ecological value can be used to solve the problems in the equivalent conversion among various ecological indexes and between ecological indexes and economic indexes. This paper calculated the ecological functions of returning farmland to forest and mountain enclosure based on the surface vegetation，compared and sequenced the relative ecological value. The results from this method are more objective and accurate because the dominant factors are captured，whereas the minor factors which are in place but cannot be used for measuring value are given up. This method can be used for evaluation of ecological values of different types of vegetation with different vegetation coverage. As a result，the headaches in evaluating the ecological value before and after the returning farmland to forest project can be solved.

Key words: returning farmland to forest, mountain enclosure, relative ecological function, relative ecological value, Jingyuan County