The Al + Si codeposited coatings were prepared on DZ417G superalloy substrates by aluminization and siliconization using pack cementation. XRD, SEM and EDS were used to study the microstructure and morphology of coatings. During the co-deposition process, the phase structure formed on the coating surface changed with Al content. In addition, the post heat-treatment also had an effect on the microstructure of coatings. When the composition (in wt.%) of the packs was xAl-4NH4Cl-4Si-(92-x)Al2O3 with different Al contents (x = 1, 4 and 6), the corresponding microstructures of the coatings were NiSi and Ni2Al3 after pack cementation, respectively. When Si content increased to 6 and 10 wt.%, the microstructure of the coatings were also NiSi or Ni2Al3. After post heat-treatment, because of Ni, Cr and Mo diffusion from the substrate, NiSi was transformed to AlNi6Si3 or δ-Ni2Si, while Ni2Al3 was transformed to β-NiAl. The Si level in the β-NiAl coatings was very small. Cr or Mo dissolved into AlNi6Si3, δ-Ni2Si and β-NiAl. The results indicate that Al content plays a role for the formations of coatings. When Al content is low in the pack, the microstructure of coatings was NiSi after Al + Si codepostion. However, when the Al content was above 4 wt.%, the microstructures of coatings were Ni2Al3 after pack cementation. Cr-rich AlNi6Si3, Cr- and Mo-rich δ-Ni2Si as well as β-NiAl coatings were formed after Al + Si codepostion followed by post heat-treatment.