For a long time, the concept of sustainability and unsustainability, the ways to assess them, and the factors involved in the process of various areas of scienc, especially the environmental ones. Sustainability in rural areas as human settlements depends heavily on a series of factors including environmental and socio-economic factors. This paper aims at introducing fuzzy set theory and the extension of fuzzy mathematical models for assessing the sustainability of rural settlements based on several socio-economic and environmental indicators. In an attempt to demonstrate the inefficiency and inappropriateness of the methods for assessing the sustainability of rural settlements, fuzzy logic as a smart system has been tried and tested for sustainability assessment. On this basis, the main question can be put forward in this way: Is it possible to assess the sustainability of the rural settlements through fuzzy logic? In an answer to this question, it can be said that the fuzzy set theory can relate the human approaches to the sustainability concept to a scientific mathematical model. To run such a model requires that the membership function be created for every single indicator in fuzzy system and serve as a basis for the operations. According to this model, whose implementation is described step by step in this paper, it is revealed that the researched settlements (Qooshkhaneh District of Shirvan) do not enjoy the same level of sustainability. The implemented fuzzy test application in assessing the sustainability level of the settlements of this district also implies that Zoo village with the rate of 0.572 has the highest level of sustainability and Sarani is the least sustainable village of the district. The research findings also revealed that there is a huge difference among the settlements of this district in terms of social and environmental sustainability. However, in terms of economic sustainability, the settlements seem more homogeneous. The reason for this could be attributed to economic potentials, environmental limitations, cultural features, and spatial situation of the villages in the region.
Key words: Unsustainability; Mountainous rural settlements; Assessment; Fuzzy logic; Qooshkhaneh district.