Continuous aggregation of socioeconomic factors is the key issue of sustainable development in urban agglomerations. To date, more attention has been paid to single urban agglomeration than to multiple agglomerations. In this paper, China's 19 urban agglomerations were selected as the case study and their spatial differences in factors aggregating ability were portrayed comparatively. Firstly, the spatial pattern of urban factors aggregating ability is relatively well distributed in all China's cases, most noticeably in the Yangtze River Delta urban agglomeration, closely followed by the Beijing-Tianjin-Hebei and the Pearl River Delta urban agglomerations. However, more significant differences on factors aggregating ability are noticeably seen between cities than among urban agglomerations. Meanwhile, the rank-size structure distribution of factors aggregating ability in China's 19 cases is in line with the Zipf's law of their urban systems, and divided into three types: Optimized, balanced, and discrete. Furthermore, the urban factors aggregation ability in one urban agglomeration is roughly negatively correlated with its primacy ratio of factors aggregating ability distribution. Lastly, urban agglomerations with higher average values of factors aggregating ability are concentrated on the three major urban agglomerations: The Yangtze River Delta, the Beijing-Tianjin-Hebei and the Pearl River Delta. Otherwise, high-high clusters in the three urban agglomerations are distinctly observed as well.Sustainability 2018, 10, 4179 2 of 20 gradually becoming vital growth poles of new-style urbanization development and the main body of China's participation in global competition, which depends on the support of continuous factor flows. Driven by "rent-seeking behavior" [8], variety of factors such as talents, technology, knowledge, information, and capital obey profit-seeking law, which leads to their continuous concentration on advantageous places [9]. As a result, the more competitive urban agglomerations tend to occupy a higher position in national urban systems and thus gain more developing opportunities. Moreover, their competitiveness and potential are accumulatively enlarged under the cyclical cumulative causal effect or Matthew effect. Therefore, the factors aggregating ability has become a key criterion to measure the sustainable competitiveness of urban agglomerations in national and inter-country competition contexts.Although factors aggregating ability is specially measured and documented by few scholars at present, much more attention is paid to theoretically refer to factors and their aggregation, such as factor endowment theory and factor flow theory. Generally, factors were classified to primary production factors (i.e., general natural resources such as land, water, forests, minerals) and created production factors (including knowledge and technology, innovation, capital and facility, etc.) by Porter [10]. Oskenbayev et al. further pointed out that the role of the first factors such as land, oil and coal i...