This examination investigates the governance of grain size on the Time-of-Flight (TOF) and speed of longitudinal critically refracted (LCR) waves in metallic materials. LCR waves are pivotal in non-destructive assessment and material characterization due to their sensitivity to microstructural variations. We conducted experiments on typical varying grain sizes and analyzed the propagation of LCR waves. The grain size of material changes when subjected to mechanical and thermal operations. In the present study the specimens of as received material cut to 100 X 50 X 8 mm dimensions. The prepared specimens first stress relieved and then subjected to the heat treatments at a temperature 440 0C, 480 0C, 500 0C, 540 0C, and 560 0C for 30 minute in vacuum furnace respectively. The TOF and speed of LCR wave measured with ultrasonic instrument and 5 MHz frequency transducers. The grain dimensions and the grain structure changes with the heat treatments. At 560 0C temperature the grains became courser and the size of the grains increased to 150µm. The grains get elongated at 500 0C and 540 0C temperature with equiaxed grains structure. The grain size of the specimens heated at 500 0C and 540 0C temperature measured as 84µm and 100µm respectively. The LCR wave speed and TOF established a linear relation with grain size effect. The LCR wave TOF and grain dimensions established negative relation, The LCR TOF smaller at higher grain size and higher at lower grain size. The LCR wave speed and grain dimensions established positive relation, with rise in grain size LCR wave speed increases and at lower grain size the LCR wave speed observed smaller. The change in ±10µm grain size changes the TOF and speed wave by ∓ 0.0005µsec and ±0.2411m/s respectively. The measurements of TOF and speed of LCR wave performed with uncertainty ± 0.000029 and ± 0.010 respectively. Our results indicate a significant correlation between grain size and both TOF and wave speed, highlighting the importance of considering grain size in ultrasonic testing and material evaluation.