The purpose of this paper is to clarify the aspects of the potential pattern of intercity associations in Korean cities in 1960 and 1975, with a view to identify the degree to which the cities do indeed operate as a system. The magnitude of direct associations and accessibilities among cities are measured by the gravity model in terms of railroad time distance and railroad distance among 35 cities. Basing on the results of direct associations among cities by the gravity model, the volume of indirect associations among cities are estimated by the largest flow. But in order to understand the underliyng potential associations, the writer also tried to use the second largest flow among cities. The main results are summarized as follows: 1. Through an analysis of the accessibilities of the 15 year period, it was proved that the promption of time distance has increased accessibilities of large center and their satellite cities to the detriment of medium and small sized centers. The result is that the promption of accessibilities has been concentrated in Seoul and the larger centers. But on the other hand, the function of the medium and small sized centers as nodal points is gradually on the decline. As expected, the accessibilities of cities increase in proportion to their population size (Figs. 1a-2). 2. The potential interaction pattern among cities on the basis of time distance and railroad distance has changed during the 15 years. In 1960, the largest flow among cities has revealed a pattern concentrating in Seoul, Busan, and Daegu. Meanwhile, in 1975, there seems a strong tendency in Seoul only. This means that with the decrease in time distance, the sphere of one day's journey is made possible in Korea, in consequence, Seoul has come to dominate the interactions with its primacy. The pattern of the second largest flow has also changed as It was found in the largest flow. In 1960, Seoul, Incheon, and Busan had divided the pattern of interactions in Korean urban system into two parts. Meanwhile, in 1975, these patterns have come to be concentrated in Incheon and Busan. As a consequence of the fact that Seoul has dominated the pattern of interactions among cities in 1975, accordingly, Busan and Incheon have come to dominate the underlying pattern of interaction among cities. When Seoul was excluded from the potential interaction among cities, Incheon develops a strong association with cities in the Joong Bu region. This means that the high primacy rate of Seoul gives rise to its prominance in interaction with all cities of Korea (Figs. 3-6). The potential pattern among cities on the basis of railroad distance is similar to the pattern as in the analysis of time distance. The difference between the results of the two datas have revealed a somewhat different interaction pattern such as the association of the cities in Jeonlanam Do, Jeonlabuk Do, and Gangweon Do with Incheon and Busan. These tendencies may be related to the change of transportation system during the 15 years (Figs. 7-10). 3. Ward's cluster analysis on the basis of time distance in 1960 and 1975 has indicated the change of groups during the 15 years. This is found that Seoul, Incheon, satellites of Seoul and industrial cities have increased their accessibilities and interactions among cities during the 15 years. In contrast, the regional administrative centers and the small sized centers have shown a pattern different from that mentioned above. An examination of the linkage tree suggests these changes well. The regional administrative centers and the satellite cities of Seoul formed one group in 1960, but the regional administrative centers and small sized centers are clustered at the earlier stages of grouping before being combined into one group in 1975 (Fig. 11). This is only a result of potential pattern among cities in Korean urban system. Through the analysis mentioned above, it is estimated that

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