Knots have an adverse effect on the mechanical properties of wood, due to influence on slope of grain, therefore wood can be less suitable for particular use. The purpose of this thesis was to examine the correlation between knots and the mechanical properties of wood and also the evaluation of different knots measuring methods. A comparison of knot measuring methods listed in the DIN 4074-1 and BS 4978 standards was carried out. The knots positions and size were measured on 475 spruce and fir structural elements with the dimension 50 mm x 150 mm x 4000 mm. Then, the global modulus of elasticity and bending strength were determined, by a destructive four-point bending test. It was established that knots affect wood strength; however, predicting wood strength based only on knots is fairly unreliable (r = 0.27 to 0.56). More reliable strength predictions on the base of knots is grading in accordance with the DIN 4074-1 standard rules for beams (parameter A; r = 0.56) and by knot area ratio grading in accordance with the BS 4978 standard (parameter tKAR; r = 0.54). The least reliable criterion is the depth of lateral knots (parameter EEB; r = 0.28). The yield of grading was higher when the knots were evaluated by BS 4978 standard; when using the DIN 4074-1 standard, however, there was too high ratio (87.4%) of underestimated elements.