With the launch of a new millenium of the 21st century ahead, one of tasks that need to be considered in urban ecological aspect is the understanding of impact of continuing urbanization on urban ecosystem, habitats, and various species of animals and plants. In addition, alternatives and measures to mitigate these impacts including environment restoration should be pursued. In this article, the following points will be reviewed; 1) the definition and significance of an eco-city and "ecological network", 2) the necessity of an eco-network as one of factors composing an eco-city, in the aspects of urban planning and urban ecology, 3) ways to build an urban ecological network and example of foreign countries, 4) as an example of a city in Korea, the present state and impending tasks of the ecological network of Seoul city, and 5) a brief conclusion and suggestion. By providing a basic tool called "an ecological network" for urban space planning, it attempts to contribute in promoting networking of cities (netropolis) and urban biodiversity. It is believed that how to create an ecological network in an artificially developed urban area is a pending issue we are faced with. Herein lies the necessity of natural environment restoration and creation based on the so-called the Third Ecology. Regarding the efforts to return the Nature to urban citizens, there is a need to view the development of a desirable urban ecological network as the starting point and finishing point of the effort. In this way, it is believed that the objectives of urban sustainability through netropolis development would also be achieved.