Abstract: Over the process of rapid urbanization, how will massive rural-urban migrants be distributed in the cities with different sizes? It is one of key issues that we must face when we set out the strategies for socio-economic development at the country level. The United States is the largest developed country in the world. In 1910, the level of urbanization in the US had reached that of today's China. Based on the census data of the US, using the Rank-Size rule, this study analyzed the evolution of the urban system of the country in the 20^th century, which may provide some implications for the development of the urban system in China and urban planning at the macro level. Results show that: between 1900 and 1970, the value of sloping index (q) presents a rising trend, which implies that the big cities grew more rapidly, compared with the small and medium-sized cities. And in the last 30 years of the 20^th century, the cities developed more evenly. In China, during the process of rapid urbanization (1985-2000), urban population in small cities grew faster than that in big cities. Comparison of the urban systems of two countries shows that the large cities in China will have a big potential in terms of urban population growth.

Key words: the 20^th century, American urban system, evolution process, implications