Fat dormouse (Glis glis) as a squirrel-like rodent and indicator of Palearctic biogeographic region is the only member of Gliridae family which is closely tied to deciduous forest ecosystem in southwestern Eurasia. Despite great studies of refugia during glacial-interglacial periods based on this indicator, there is no documented data on its biological and ecological characteristics in Iran. Our results indicated that Fat Dormouse in Iran is the largest species of all known dormice in the world. It has the largest number of nipple count (n: 14). In the breeding season, male’s aggregation behavior and movement is increased and they approach female groups around. After successful mating in the early August, pregnant females will breed about 30 days later in early September. Mean litter size (n=9) was estimated 7-9 offspring. Our analysis regarding to the habitat use showed that feeding items and tree crown connectivity are the most affecting factors.