The sound velocity in a fibre shows a pronounced anisotropy，that is，the sound velo-city along the axis of an oriented fibre is much greater than that perpendicular to it，and therefore it may be used as a measure of the molecular orientation a in the fibre， α=1-C2u-C2 where C and Cu are velocities of sound along an oriented fibre and a completely un-oriented fibre respectively．The apparatus used consisted of a pulse generator which sends a pulse of 70~tsec duration with a repetition frequency of 100 times per second to the moving coil of a loud speaker．The fibre was coupled to the vibrating membrane through a spring clamp．When exited by the pulse，the system underwent damped oscillation with a natural frequency of about 4000 Hz．At the other end of the fibre，about 200 cm from the clamp，a crystal pick-up was located，the output of which was sent to an oscilloscope with the sweep circuit triggered by the pulse voltage．The time of propagation of the longitudinal vibration along the fibre from the clamp to the pick-up was observed from the oscilloscope record，and the sound velocity in．the fibre calculated． Since the mechanical properties of fibre depend considerably on the moisture regain and tension，the effect of relative humidity and tension in the fibre on the sound velocity were studied．The best choice of the experimental conditions for the determination of molecular orientation by the sound velocity method appears to be the following：room tempera-ture，28—30℃；relative humidity，60％；and under a tension somewhat below the cri-tical Young’s modulus of the fibre．The sound velocities in the fibres of different draw ratios (r) were determined and extrapolated to r=1 to give the value for Gu Experimental results indicate that the degree of molecular orientation of monofils of Nylon 6 increases to approximately 80％ after drawing and after-treatment．The degree of orientation for several products of Nylon 6 fibre such as yarns，mono-fils，and cords was determined．Fibres that possess greater breaking length show higher degree of orientation．