Climate change is the most serious problem we face in the 21st century. Accompanied by the rapid expansion of cities worldwide, the phenomenon of urban heat islands has become a critical environmental issue in many large cities, since it is associated with a warmer climate. An exposure to higher temperatures can result in a spectrum of outcomes in the realms of human health and well-being. These outcomes range from mild discomfort to life-threatening medical conditions. Fortunately, trees and vegetation in the urban environment help to cool urban climates. This occurs through shading and evapotranspiration. Although there have been numerous studies on microclimate-adjusting effects of urban green spaces, few of these studies quantify the economic value of air temperature reduction provided by these spaces. This is especially true for cities suffering from the heat island effect. Urban heat island conditions have been observed for more than half a century in the city of Beijing. The area affected by this heat island rapidly expanded from 111 km² in 1987 to 292 km² in 2009, and the percentage of heat island to built-up area increased sharply from 11% to 27%. Therefore, it is very important for city managers to estimate and achieve economic benefits of the cooling effect of green spaces. Based on a number of empirical studies and inventory data concerning urban green spaces in Beijing (2009), this study estimates the economic effects of the city's green space on air temperature reduction through evapotranspiration. The results show that green areas in Beijing could reduce the temperature by 0.8-4.8℃. As a consequence, urban green spaces totaling 6.1×10⁴ hm² in built-up areas of the city could absorb 4.61×10¹⁵ J of heat via evapotranspiration during summer. The average heat absorbed by these spaces could reach 8.4×10⁸ J·hm^-2·d^-1, which corresponds to the cooling effect of ten 1000-W air conditioners. According to the price for residential electricity, the summer temperature reduction from the urban green spaces could save 641 million RMB of the cost of electric energy consumption per year. This figure is based on annual average savings reaching 1.05 RMB/m² for urban green area with temperature reduction. In addition, the temperature reduction effects of urban green spaces varied with type and location. The amount of heat absorption of protected green zones was significantly less than those of public, roadside, affiliated, and productive green spaces. Average values of air temperature reduction per unit area of green space in Dongcheng District, Xicheng District, Miyun County, and Yanqing County were significantly higher than those of the districts of Pinggu, Fengtai and Shijingshan. These cooling effect variations may be related to differences in area and structure of urban green spaces. Therefore, evidence is provided to support the commitment of resources to urban green infrastructure enhancements. Furthermore, such evidence conveys the importance of the structural characteristics of green land. Urban foresters, ecologists, and landscape planners should take advantage of this type of data to plan, design and manage green spaces in heat island areas. In this way, communities can realize energy savings, and cities and urban neighborhoods can be made more livable.