[en] A new informative criterion - shape factor - is proposed for characterizing the nature of flaws. Formulas for calculating this criterion are given and the results of experimental checks are presented. Volume and planar defects in the cross-section of a weld joint are shown to differ by their respective shape factor. The relation is investigated between this difference and the depth of the flaw occurrence, the attenuation factor of ultrasonic oscillations and characteristics of the flaw detector. A cumulative table with formulas for calculating the shape factor is given and data obtained in measuring the shape factor Fsub(s) of real flaws of various types are presented