SA508Gr.4N steel is commonly used in nuclear pressure vessels. The current study discloses the impact of austenitizing temperature on its martensitic transformation. Various austenitizing temperatures were employed to examine the martensitic transformation at different experimental conditions. The characteristics of grain boundaries, microstructure, transformation kinetics, and crystallography of the martensitic phase following the austenitizing process and martensitic transformation were analyzed. Austenitizing temperatures had minimal effects on the critical temperature of the austenitizing phase transition. However, an increase in austenitizing temperature resulted in larger grain sizes. The phase composition primarily consisted of lath martensite, accompanied by a small amount of residual austenite (RA). As the austenitizing temperature increased, the temperature of martensite-start temperature (Ms) initially decreased and then increased. On the other hand, the temperature of martensite-finish (Mf) showed no significant sensitivity to changes in the austenitizing temperature. Moreover, the average size of the lath martensite structure was increased, and there was an increased tendency for variant selection as the austenitizing temperature increased, and the combination of specific orientation relationships appeared. This study is very valuable for revealing the microstructure evolution at different temperatures.