Abstract: The regeneration structure and biodiversity of trees and shrub species in under story of pure and mixed Oak plantations were investigated in Chamestan Forest and Rangeland Research Station of Iran. Planted species including Oak (as main species) and Hornbeam (as associated species). This species were planted in five proportions (100Q, 70Q:30C, 60Q:40C, 50Q:50C, 40Q:60Z) in Northern of Iran in 11 years ago. All of regenerating seedlings and saplings of woody plants were divided into two height classes of 15-200 cm and more than 200 cm. In biodiversity study dominance index of Berger-Parker, diversity index of Fisher alpha, richness index of Margalef and evenness index of Equitability J were used. The results showed that abundance and diversity of regenerated species in under story of all plantations of oak were more than unplanted control plots and also under 50Q:50C were more than pure stand of oak. The presence of primary forest species in under stories of mixed plantation showed the usage of these plantations in development of succession in natural forests.