This study was investigated the effect of austenitizing treatment the microstructure and mechanical properties in modified 440A steel, and the results were as follows. The amount of remaining carbide decreases with increasing the austenitizing treatment temperature, and all carbide is completely dissolved at $1250^{circ}C$. The amount of remaining carbide decreases with increasing the austenitizing treatment time, but the carbide remains insoluble up to 120 minutes at $1050^{circ}C$. The strength and hardness gradually decrease with increasing the austenitizing treatment temperature and is significantly lower at $1250^{circ}C$, while the elongation and the impact value rapidly increase. The strength and hardness rapidly decrease, the elongation and impact value rapidly insrease with increasing the austenitizing treatment time and exhibit no change at above 120 minutes. The austenitizing treatment modified 440A steel is required for temperature of above $1050^{circ}C$ and time of above 60 minutes.