China has experienced unprecedented population mobility since 1978, creating history’s largest flow of rural-urban migration in the world. This paper attempts to assess the role of the city-ward migration in China’s urbanization in 1978–99, and to empirically investigate factors behind the migration boom with time-series and cross-sectional data. We find that (i) rural-urban migration made dominant contributions to Chinese urban population growth; (ii) while moving together with Chinese economy, the migration was caused by the economic growth, and not vice versa; (iii) interprovince migrants were encouraged by the rural-urban income gap and discouraged by their geographic distances to destinations; and (iv) the amount of intraprovince migrants is negatively related to the level of urbanization in that province.