As human society has entered the century of the city, studies of urban ecosystems have shown a booming increase since the beginning of the 21st century. Urban ecology aims to fully understand the problems and find the solutions to sustainability challenges of an increasingly urbanized planet. As typical representatives of developed and developing nations,the U.S. and China are experiencing different urbanization processes. Yet the development of scientific investigations in the field of urban ecology in the two countries seems to have reached the same goal by different means. We compare the development of urban ecology in the U.S. and China in terms of related disciplines, main research fields, and hot topics. In general, American urban ecology connects tightly to a basic ecology research paradigm and trans-disciplinary study, while Chinese urban ecology emphasizes practical problem-solving but lacks sufficient theoretical and technical support. In the end, four key differences between American cities and Chinese cities are discussed, including urbanization stage, urban boundary and population, urban form and landscape, and urban metabolism and management. These differences present both challenges and opportunities for collaboration in urban ecological studies between U.S. and China. The collaboration between urban ecologists of the U.S. and China will contribute not only to the two countries’ cities but also to cities from both developed and developing countries.