In Boso peninsula, evergreen broad-leaved forests zone, sika deer population has expanded its distribution since 1970s. The center of distribution reached to 30-40 deer/ km² by 1991. Associated with the population increase, jaw size of calves and body mass and fat reserves of females decreased. However, no obvious mass mortality occurred. Evergreen leaves and acorn were the primary food in winter of Boso deer. The both evergreen leaves had relatively high and stable crude protein contents throughout the year. The annual acorn production did not affect the deer density. The fecal nitrogen of the Boso population was lower than that of the northern population in the deciduous broad-leaved forests in summer, while higher in winter. Thus, the magnitude of the seasonal fluctuation was relatively smaller in the Boso population than those in the northern population. The density dependent pregnancy rate was found in adult females and it decreased when the deer density exceeds 15 individuals/ km ². The age-specific survival rate has not been obvious difference between calves and adults. The most sensitive parameter to increasing density was reproductive rate firstly, followed by survival rate, which is opposite sequence in northern sika population and other cervids.