Tending of mixed oak-beech stands, where one of the trees is shade bearer and the second one is light demander is very complicated due to the significant variability. The best way of tending mixed stands is crown thinning with positive selection. The objective of this thinning should be to create the most favourable conditions for the development, increment and the stability of future mature trees, thus creating an optimal structure in terms of quantitative and qualitative production. The aim of this thesis was to find out the long-term effect of thinning on the structure of mixed oak-beech stand. The research was carried out on university forest enterprise, in compartment 514, in based permanent research plot for this purpose. There are four permanent research plots with varying proportions of oak and beech target trees, one of which serves as a control plot where no thinning have been carried out, and the quality crown thinning with positive selection has been applied to the remaining three. Data on the diameter and positions of individual trees were obtained on the plots. Within these plots, differences in absolute frequency, basal area of the stand, growing stock, and average tree volume and diameter values were assessed. The level of structural diversity of stands was assessed between plots using the homogeneity coefficient and the structural complexity index, and the species composition is characterized by using the relative important value index. This assessment was carried out both at the level of the entire plot and at the level of the target trees on each plot. Target trees on the affected plots were significantly higher than the trees on the control plot. Similarly, the growing stock, basal area or volume of individual target trees on the tending plots were higher than the control plot. The most varied area was the non-tending plot, but the tending plots were also significantly spatially differentiated. Using the homogeneity coefficient, it was found that the plots tended by the quality crown thinning are more heterogeneous than the non-tending plot. The results show that the crown thinning with positive selection affected the stand structure.