Abstract: By investigating gap generation of virgin fir forest on Mount Sejila, the size and the generation of gaps, relationship between the age and the regeneration of gaps were studied and the qualitative characteristics and the growth up of seedling, young tree and small tree between gaps and non gaps were compared. It was discovered that the gaps with area about 100 m2 is the most appropriate for seedlings lower than 20 cm in height to exist; if the area of gap is over 100 m2, the amount of seedling reduces with the increase of gap area; the gaps with area about 300 m2 is the most appropriate for young fir and small diameter tree to grow. There is the most abound seedlings and young trees and less small diameter trees in the gaps ageing about thirty, the quality of small trees increases while the quality of young seedlings and trees decreases with gap age increasing. The density of seedling, young trees and small diamater trees in gaps is higher than that in non gaps. Gaps have little effect on growth of seedling aged three while they promote the growth of young trees aged four, especially they have obvious effect on growth of young trees aged from 9 to 16.