Ecological Psychology and Micro-Sociology studies concerning behavioral-milieu systems have created an important basis for evaluation and prediction of the performance of built environments. In this context, the Behavior Setting Theory introduced by Roger Barker in 1968 defines the complicated behavioral-milieu framework or synomorphy as the determining factor of the environmental behaviors of people. Therefore, comparing synomorphy of various behavioral settings will help enhance the performance of built environments. Understanding the interaction of environment and behavior, behavioral affordances of the built environment, and behavior occurrences in coordination with the ecological environment are the aims of this research. This research is a comparative study of the Tajrish Historical Bazaar and Arg shopping center in Tehran, Iran. This applied, descriptive-analytical research uses a qualitativeـquantitative approach, with data collection through surveys, ethnography, and fieldwork techniques such as observation without intervention. Data analysis is done through deductiveـinductive reasoning. The result of eight surveyed behavior patterns shows that regarding "quality of mobility", "affordance of hosting behavioral diversity and change over the time", "quality of social interactions", and "quality of behavioral control", the two behavior settings differ. Comparative study of 28 parameters of milieu structure and 4 parameters of social structure in these two ecological environments point to behavioral mobility, diversity of innovative behavioral strategies, and passionate and brief human interactions in the Tajrish Bazaar, whereas to less mobility and reduction of selective and social behaviors in Arg shopping center. This research concluded that the Tajrish Bazaar is high in synomorphy, which has resulted in the development of ongoing and innovative interactions between people and the ecological environment, while Arg shopping center is low in synomorphy, resulting in the limited diversity of environmental behaviors.