This analyzes the relationship between urban compactness and CO 2 efficiency in China. A quantitative indicator of urban compactness and CO 2 efficiency (CO 2 economic efficiency and CO 2 social efficiency) was selected and calculated using remote sensing and panel data analysis. According to data from 30 cities, urban compactness is correlated positively with urban CO 2 economic efficiency, which is measured by the ratio of GDP to total CO 2 emissions. Conversely, the compactness ratio shows a negative correlation with urban CO 2 social efficiency, which is measured by the ratio of social welfare indicators to total CO 2 emissions. These results indicate that, within China, designing compact low-carbon cities could lead to an increase in urban CO 2 economic efficiency. However, if the compactness ratio is too high, the increased compactness accompanied by various socio-environmental problems will result in a decrease in urban CO 2 social efficiency. Therefore, compactness entails a trade-off and this should be considered by policymakers.