Single crystal of diamond, LiNbO3, LiTaO3, Silicon, and various polycrystalline hard materials were used to measure the room temperature ultrasonic velocity utilizing a pulse-echo method. The elastic constants were then calculated from the wave velocities. The results indicate that[I 11] direction of diamond possesses the fastest longitudinal wave velocity (19 039 m s(-1))and the largest Young's modulus (1223 GPa). Acoustic wave velocity and Young's modulus of polycrystalline CVD diamond are similar to those of single crystal diamond, but much larger than Co or SiC bonded polycrystalline diamonds. The existence of grain boundary at the polycrystalline CVD diamond does not significantly affect the propagation of the acoustic wave. Cubic boron nitride (cBN), silicon and diamond possess cubic structure, however, the wave velocity and elastic constant of cBN and Si are inferior to diamond, due to the differences in the bond nature and bond length. The dependents of acoustic wave velocity and elastic constant on the crystal structure as well as specific gravity are discussed. (C) 2004 Elsevier B.V. All rights reserved.