Abstract: In this study, reproductive biology of Glis glis (Linnaeus, 1766) and weight gaining of pups were studied in four localities by using wooden nestboxes and tattoo pliers method in mixed deciduous forest of Istranca Mountains in Turkish Thrace. By considering morphological peculiarities, body weight gaining and reproductive signs of adults, mating behaviors and breeding times were measured from specimens captured in nestboxes. Results showed that females were mated between June 15 and August 18 and gave a birth between July 14 and September 16. The litter size ranged from 1 to 12 (Mean 6.05, n = 100) and the least weight of newborn is 2 g. Female to male ratio were in the range of 48.2 to 51.8%, respectively (n = 82). Over wintering adult yearlings reached maturity in mid June. Yearlings arising hibernation after May gained weight during September and October and they entered hibernation at the end of November. The litters at bird gained weight daily from 0.83-1.19 g and reached to 25.16-35.7 g at mean within 30 days during August and September.