Aluminide coatings are known to protect steel at elevated tempeeratures in sulphurous, fused salt and marine atmospheres as well as providing satisfactory wear and scaling resistence[1]. Mild steel samples were pack-calorized under same conditions(950℃, 4 hr) with different particle size of pack powder and the calorized specimens were characterized by XRD and SEM. Results showed that hte thickness of coating layer and quality were improved as decreasing the particle size of pack powder. Especially, diffusion coating layer was formed with 200 mesh(75㎛) particle of pack powder.