This paper describes an objective research method for the evaluation of cities in aesthetic terms. The evaluation in the research is carried out via urban skyline scale. The aesthetic value is brought into numerical quantity by adapting the visual codes of the design elements that compose the urban skyline. The connections and correlations between the visual codes represent the information capability of the urban skyline. The relation and the fictional order of the structure and landscape integrity, composing the urban skyline, define the aesthetic qualities of city. Entropy, according to the information theory, is the quantification of the information amount of the coded data. Skyline coding models connected to the formal aesthetic design criteria with the developed entropy approach are to be discussed in terms of their contributions to urban design researches. In this research, the skylines of Istanbul's various urban areas are evaluated by using entropy as an innovative method in urban design applications. For obtaining measurable quantitative aesthetic values for urban skylines, these values have been compared with each other. In accordance with this, it has been determined that Istanbul's urban identity attributes and skylines that represent its historical structures have more informational aesthetic value than other urban patterns. So, there is proximity between quantitative values of entropy and informational aesthetics. It has been proved that, based on formal aesthetic criteria, aesthetic qualities of cities are measurable with the entropy approach.