The paper is dedicated to search for an ultrasonic sensing method to identify the state of a material that makes it possible to classify the structure to be diagnosed as the structure of which state is associated with occurrence of plastic strains. We examined the effect of C275 steel’s uniaxial strain-stress state on velocity of an elastic Rayleigh wave. Elastic waves were generated by piezoelectric transducers using oscillation frequencies of 2, 5, and 10 MHz. Transmitting and receiving transducers have been fitted in a single block and permanently spaced 50 mm apart. The transmitting transducer was excited using А1214 test instrument. The sensing pulse was taken up using Tektronix TDS2022 oscilloscope with maximum time resolution of 2 ns. Delay of the received signal relative to the initial position of the informative point (pulse’s zero crossing) has been chosen as a numerical indicator of elastic wave velocity variations caused by strain. Rise in delay value at permanent space between transmitting and receiving transducers is indicative of decrease in sensing pulse velocity and vice versa. For pure elastic strains the difference between delays (velocities) of the waves propagating parallel and perpendicular to stress acting in a material under load is proportional to the acting stress. For plastic strains the dependence of acoustic anisotropy (difference between delays) on the applied stress is non-linear and drops with rise in stress. Delay variations in the elastic range do not depend on the sensing pulse frequency. As to the plastic strain range we observe a significant dependence of velocity on frequency. Due to the results obtained we propose to use the dependence on Rayleigh wave velocity as a diagnostic symptom of a material under plastic strains.