Abstract The fatigue crack length a and the changes of ultrasonic attenuation △α at frequency 10 MHz 011 compact test sample of 30CrMnSiNi2A steel have been simutane-uosly measured as a function of fatigue cycle number N. It was shown that the △α(N) curve was in good response to the a(N) curve and the △α was traced out during the period of crack nucleation. The ultrasonic attenuation a as a function of load P in pulsating tension fatigue test has been investigated. When P
c, where Pc is a critical load, a decreases wth increase of the load; when P>Pc, a increases rapidly with increase of the load. A depinning model of dislocations is suggested. The ultrasonic attenuation is a very useful tool for studying the growth of fatigue crack.