Abstract: Two kinds of thermal barrier coatings(TBCs) with different porosities at ceramic-coat and ceramic-coat/bond-coat interface by plasma spraying (PS) technology with NiCoCrAlY as bond-coat and 8wt% Y₂O₃ stabilized ZrO₂ (8YSZ) as ceramic-coat. The difference of thermal cyclic lifetime of TBCs were studied, and the failure mechanism of TBCs with different porosities was analyzed. The distribution of stress in TBCs was calculated with finite element simulation, and the formation reason of the repeated parallel cracks in the porous TBCs and the failure mode of the two TBCs peeling were also analyzed. The cross-section microstructures and element distribution of TBCs were analyzed by using optical microscope(OM), SEM and EDX. The results show that the porous TBCs have a double longer lifetime than the dense TBCs. For the porous TBCs, as a consequence of the more pores and micro-cracks in ceramic-coats and interface to release the accumulated strain energy within the TBCs. At this time, the repeated parallel cracks in alumina layer further decrease the stress between ceramic-coats and bond-coats, which can prolong the lifetime of porous TBCs. Foundation for the structure design of the longer lifetime TBCs was laid.