The research tries to provide an approximation through a comparative analysis between six cities of Reykjavik, Malmö, Vancouver, Seoul, Copenhagen, and Singapore that have obtained the reputation of "Green Cities", trying to represent a significant difference in terms of geographical location, territorial extension, population, etc., extracting the most representative indicators or variables and comparing them with one another. Therefore we can make clear the guidelines to identify indicators that allow us to define a city as a green city. What we seek is to counteract different measures carried out to be able to extract from each one of the common factors on which significant achievements have been made to make them more sustainable cities, fit for life and that have led to international recognition. We have many examples of the classifications of characteristics or indicators; of which we could cite the most relevant, such as air pollution, noise, waste management, and alternative energy, mitigation against climate change. The research focuses on several aspects: in the field of transport, in the field of urban planning, in the field of the environment, in the field of renewable energies that allow identifying with clarity the progress in terms of mitigation measures that these cities have implemented. The personal aspiration is to make clear how the connotation of green cities is combined, under the guidance of the ecological city theory, the theory of sustainable development, to analyze the basic situation and the practice in the planning and construction of green cities, to summarize the results and problems, experiences and lessons, and present the inspiration of various cities in the planning and construction of green cities, in the hope of providing a reference for other cities that have this common goal.