Abstract: A method of obtaining the pure mode direction of the trigonal system by the slowness surfaces is introduced. By the slowness curves of propagation normal to the Z-axis and propagation in the YZ and XZ planes, we can easily find the direction that both the energy velocity and the phase velocity direct to. This crystal direction is the pure mode direction.

Key words: A method of obtaining the pure mode direction of the trigonal system by the slowness surfaces is introduced. By the slowness curves of propagation normal to the Z-axis and propagation in the YZ and XZ planes, we can easily find the direction that both the energy velocity and the phase velocity direct to. This crystal direction is the pure mode direction.