This study quantifies the influence of green spaces on microclimate and PM10 concentration in a typical residential district of Nanjing (China) by employing the CFD-based and microclimate model ENVI-met 4. Five green indices, related to quantity and structure of vegetation, are employed to investigate the impact of different types (grass, shrub and tree) and layouts of a green space located in the center of the residential district under an average Nanjing summer day. Results show that the thermal comfort (expressed by the mean radiant temperature MRT and the predicted mean vote PMV) is slightly enhanced with increasing green quantity, especially trees, even though more trees may increase the wind blocking effect with a consequent slight increase of pollutant concentration. In this regard, a single patch of trees located in the central part of the green space is preferable. The green indices are shown to be useful for studying the relationship between green space morphology, microclimate and air quality in cities.