YSZ TBCs with the initial TGOs of different thicknesses were subjected to isothermal cycling test to examine the relationships between the thickness of initial TGO and TBCs isothermal lifetime.. The TBC system consisted of Inconel 738 substrate, cold-sprayed NiCoCrAlTaY bond coat (BC) and 8YSZ topcoat. The TBCs with different thicknesses of TGOs were prepared through pre-oxidation. Results indicated that the lifetime of APS TBCs was significantly influenced by initial TGO thickness. As the TGO thickness increased from 1.3 μm to 6.5 μm, the lifetime of TBCs decreased from 124 cycles to 11 cycles. It was found that there is a critical TGO thickness of 5 μm over which TBCs fails rapidly. Moreover, it was confirmed that TGO thickness also affects the failure mode of TGO. When TGO thickness is less than the critical thickness the cracking takes primarily place along the YSZ lamellae interfaces, which when TGO exceeds the critical thickness the cracking occurs through TGO or the interface between the TGO and YSZ coating.