Urbanization is the inevitable trend of social and economic development of people. One of the tasks of urban planners is to plan and design urban areas for individuals and households. More than half of the world's population lives in urban areas. Complexity is an important factor in urban areas because of natural hazards, emergency or other critical situations, waste of time, common using of the landscaping, sport facilities and other social areas etc. Additionally, complexity affects the spatial distribution of residential buildings, industry, commerce, energy consumption, water use, waste management and other environmental and social correlations. An important goal in future is to make existing and new urban areas more aesthetic, self-sufficient, sustainable and enjoyable places for better living of people while interacting with each other. Entropy can be used as a criterion for the quantitative measure of spatial information on maps. Such a quantitative criterion can be used to compare the spatial distribution of the real objects like residential, recreation, social and sport facilities in urban areas. In this study, metric entropy method was experimented and applications were carried out by a case study in Istanbul for defining the complexity of urban areas. Moreover, a quantitative measure is calculated to define the complexity of urban areas. Thus, existing or planned urban plans will be evaluated by the method and significant results obtained were discussed.