Microstructural changes and oxidation resistance of simply aluminized Ni-based single-crystal superalloys were systematically investigated. There are several kinds of microstructural changes observed in the as-aluminized Ni-based single crystal superalloys. For instance, in the case of a gritblasted specimen, secondary reaction zone (SRZ) was formed in the vicinity of substrate/coating interfaces. On the other hand, in the polished specimens, voids were formed by thermal cyclic heating. It is also found that grit-blasted specimens with SRZ formation exhibited accelerated oxidation. Microstructural changes are slightly accelerated along 〈110〉 direction rather than 〈100〉 direction. These experimental results can be explained by the recrystallization of substrate surfaces and subsequent interdiffusion between the coated layer and the substrate. Cyclic oxidation tests were also performed to assess the oxidation resistance of specimens with different microstructure and it is found that specimens with SRZ formation demonstrated worse oxidation resistance than those with voids formation, probably due to the difference in interdiffusion of solute elements between aluminized coatings and substrates.