The effect of ultrasonic vibrations on the austenite to martensite transformation in 304L stainless steel was examined by passing powerful acoustic ultrasonic energy into the specimen before transformation. The results indicate that ultrasonic treatment of 304L stainless steel influences the austenite to martensite transformation by activating the austenite. This activation has been attributed to the increase in dislocation density and point defects which enhance the number of martensite nucleation sites on ultrasonic treatment. © 1988.