The site index conversion equations are important for estimating the site index of one tree species from the site index of another tree species in mixed stands. In this study, data were obtained from 162 sample plots in mixed stands of Scots pine (Pinus sylvestris L.) and Oriental beech (Fagus orientalis Lipsky) from the Black Sea Region in the north of Turkey. The breast height ages and height of the site trees were measured and the site index was estimated by these data. Geometric linear regression was used to estimate the parameters of a linear model relating to site indices of Scots pine and Oriental beech species. The correlation between the site indices of Scots pine and Oriental beech was 0.818, indicating a good fit to the linear model. Validation procedure showed that conversion equations were suitable for the studied mixed stands. The conversion equations can be applied to mixed species stands with inconsistent stratification or in stand conversion situations for an accurate and reliable evaluation of forest quality.