Summary This study was designed to investigate the efficiency of flake's alignment and orthotropic properties of oriented strand board (OSB) by using ultrasonic wave method. It was designed also to discuss the correlation among ultrasonic velocity (V), dynamic modulus of elasticity (Ed), modulus of elasticity in bending (MOE) and modulus of rupture (MOR). Results are summarized as follows: The flake's aligned angle (θ): the angles between flake length direction and mechanical alignment direction on the face layer of OSB within 0°–30° were 93–97% for JSP(OSB made from 100% of Japanese cedar flakes), JMP(OSB made from 50% of Japanese cedar flakes and 50% of Taiwan paulownia flakes) and CMP(OSB made from 50% of China fir flakes and 50% of Taiwan paulownia flakes) groups OSB, and on the bottom layer, 76–85% for these three groups OSB, respectively. The ultrasonic velocity (V) at 0° of angle between flake's length direction and mechanical alignment direction (V0) was the highest, decreasing rapidly with increase of aligned angle (θ). The lowest value of ultrasonic velocity occurred at 90° of angle between flake's length direction and mechanical alignment direction (V90) and the relationship between θ and V could be represented by Hankinson's formula, and Jacoby equation. The orthotropic properties of OSB, defined as the ratio of V0/V90 were 2.4–3.2 for the full piece specimens, and 2.4–2.6 for the bending tested specimens. Interrelationships between V, Ed, MOR and MOE were positive linear regression formulas.