Abstract: The urban spatial expansion has led to the considerable substitution of natural vegetation-dominated land surfaces by impervious surfaces, especially in large cities, with great impacts on urban ecological quality. Two most heavily populated cities, Shanghai of China and New York of USA, were chosen as the study cases. Based on Landsat images obtained in 1989, 2002, 2015 in Shanghai and in 1991, 2001, 2015 in New York, normalized difference impervious surface index (NDISI) was used to extract impervious surface (IS) information. The remote sensing based ecolo-gical index (RSEI) was then applied to evaluate the changes of urban ecological quality caused by the increased impervious surface. Furthermore, landscape pattern indices were used to analyze the differences of spatial structure of impervious surface between Shanghai and New York and their influences on urban ecological quality. The results showed a significant difference in urban expansion rate and pattern between Shanghai and New York from the early 1990s to 2015. The IS expansion area in Shanghai was 17.4 times as much as that in New York. The annual IS increase rate of Shanghai was 62.2 times as much as that of New York. Shanghai had experienced an expansion from urban center to the surrounding countryside in a concentric ring pattern, whereas New York showed no much expansion but had IS increase mainly within the inner city through space filling pattern. These differences in IS change rate and spatial distribution pattern had resulted in the difference in urban ecological quality of the two cities. The mean RSEI in Shanghai dropped from 0.717 in 1989 to 0.453 in 2015, with a decrease of 36.8%. In contrast, the RSEI of New York had a decline of 6.9% from 0.552 in 1991 to 0.514 in 2015. The poor ecological condition urban area tended to have large IS patches that were well connected and aggregated.