Basic mechanical properties, modulus of elasticity and bending strength must be determined in order to classify structural timber in terms of strength. Bending characteristics are obtained from bending moment diagrams. They are used for prediction of the timber quality in the strength classes. The Master thesis was focused on the analysis of bending moment of beech timber. The specimens with the dimensions (50 x 150 x 300 mm, N = 48) were tested using the destructive bending method according to the standard STN EN 408. Stress - strain curve was created for each specimen and the strength class was determined. Measured modulus of elasticity Em,g (not Em,0,mean) was used to determine the strength classes. The influence of the structure of beech wood as well as defects, moisture content, shape changes on the course of bending moment diagram was proved in all phases (elastic, elastic – plastic, with unpredictable failures). Despite the low number of specimens in the population (N = 48), the average values of the slopes were different in the individual classes: S = 482 (D30), S = 519 (D40), S = 571 (D50). An increase in the values in individual strength classes was also recorded fo Em,g and fm. However, the reliability of the prediction of bending strength from modulus of elasticity in beech wood is significantly lower than in spruce.