The acoustoelastic theory states that mechanical stress relates to the wave speed. The microstructure of the materials influences the propagation of any ultrasonic wave, which is a major drawback in employing critically refracted longitudinal waves (LCR) in field measurements. The present study investigates the effect of mean austenitic grain size (MAGS) on propagation speed of LCR waves in ASTM A36 low carbon hot-rolled steel plates subjected to different heat treatment temperatures. The samples were heated at 900, 1000, 1050, 1100, 1200 ∘C for 30 min to obtain different grain sizes. They were measured as received and after the heat treatment, employing the ultrasonic method. The MAGS were compared to the grain size obtained from optical microscopy. The results confirmed the influence of the MAGS on the LCR speed, which can be represented by a second order polynomial curve. From the experimental results, we show that it is necessary to correct the effect of the MAGS on the LCR speed; otherwise we cannot measure the stresses without previous calibration using a stress reference Ver menos