Coenological based diversity examinai ons were carried out in the Börzsöny off set on the sessile oak dominated stands. To reveal the sylviculture’s eff ects on the biodiversity, in 6 forest stands, characterised by same standard parameters were carried. The Shannon- and Simpson- diversity indices were used for comparison of biodiversity values of diff erent age-groups. Altogether 88 vascular plant species (15 trees, 11 shrubs, 62 herbaceous taxa) were ideni fi ed in the stands of sessile oak woodlands. The diversity of the canopy level was the highest at the 19 year-old stand, and the lowest at the oldest, 92 year-old stand. The diversity of the shrub layer was the highest at the 61 year-old and lowest at the 2 year-old stand. In case of the ground level increasing diversity values were noi ced between the 2 and 61 years old groups, then the diversity values decreased at the stands which are older than 61.