The paper reports results of measurements of the length of anatomical elements and wood density in air dry state and ultrasound wave propagation velocity along the grains in the birch wood obtained from 30-year-old plantation. Variations of the anatomical elements' length and wood density along the grains have been found similar and possible to approximate by power functions of similar correlation coefficients. The propagation velocity of ultrasound waves has been found to be a linear function of the anatomical elements length and wood density and the correlation coefficient characterising the relation with the anatomical elements length has been much higher than that describing the relation with wood density. It has been shown that the ultrasound wave propagation velocity is determined first of all by the length of the anatomical elements and not by the density of cell walls, i.e. the wood density.