Hot corrosion is one of the main destructive factors against thermal barrier coatings (TBCs) applied on gas turbine hot components. In this study, hot corrosion behavior of conventional bilayer and compositionally graded layer (CGL) TBCs in two different micro- and nanostructures was evaluated. For this purpose, nanostructured and micro-Y2O3-stabilized ZrO2 (YSZ) were used as ceramic powder feedstocks. NiCrAlY metallic bond coat was deposited on IN-738LC nickel-based superalloy by air plasma spray (APS). Two groups of compositionally graded layer TBCs consisting three layers of NiCrAlY and YSZ in the weight ratios of 100:0 (bond coat), 50:50 (gradient layer) and 0:100 (top coat) were prepared by air plasma spraying. In one group of CGL-TBCs, micro-YSZ feedstock was used, and in the other one, nanostructured YSZ feedstock was used. Bilayer TBCs consisted of two layers (bond coat and top coat). Micro-YSZ and nanostructured YSZ powder feedstocks were applied for conventional and nanostructured bilayer TBCs, respectively. Hot corrosion studies were performed on the surface of the mentioned four TBC groups in the presence of molten mixture of V2O5 + Na2SO4 at 900 degrees C for eight-hour cycles. At each cycle, salt concentration of 4 mg/cm(2) was used. Microstructural evaluation, elemental and phase analysis were performed using field emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD). Based on the results, the hot corrosion resistance of nanostructured bilayer TBC was improved by 2.5 times compared with that of micro one. Furthermore, compositionally graded layer TBCs exhibited superior corrosion resistance compared to the bilayer TBCs. It was also revealed that applying gradient layer TBC compared with nanostructured bilayer, one has greater effect on increasing the hot corrosion resistance of conventional bilayer TBC. Moreover, a combination of compositionally graded layer and nanostructure TBCs resulted in higher corrosion resistance. In addition, hot corrosion mechanisms of nanostructured and micro-TBCs were suggested.