Abstract: LaMgAl₁₁O₁₉ thermal barrier coatings (TBCs) were prepared by atmospheric plasma spraying. The crystallization behavior of the coatings and the synthesis mechanism of LaMgAl₁₁O₁₉ powders were researched. The results showed that the plasma-sprayed coatings contained some amorphous phase, and LaMgAl₁₁O₁₉ powders were partially decomposed into Al₂O₃, LaAlO₃, and MgAl₂O₄ in the plasma spraying process. The amorphous phase was recrystallized at a temperature of approximately 1174.9°C, at which level the decomposed Al₂O₃, LaAlO₃, and MgAl₂O₄ reacted again. The resynthesis temperature of LaMgAl₁₁O₁₉ in the plasma-sprayed coatings was lower than that of LaMgAl₁₁O₁₉ in the original raw powders. The synthesis mechanism of LaMgAl₁₁O₁₉ powders can be summarized as follows: during the first part of the overall reaction, La₂O₃ reacts with Al₂O₃ to form LaAlO₃ at approximately 900°C, and then LaAlO₃ further reacts with Al₂O₃ and MgAl₂O₄ to produce LaMgAl₁₁O₁₉ at approximately 1200°C.