Grain‐boundary mobility of the plasma sprayed aluminum oxide (Al2O3)–carbon nanotube (CNT) composites is evaluated in the current work. Grain mobility is evaluated from the grain growth within the spray‐dried particles and thermal history experienced during high‐temperature plasma processing. CNTs form an interfacial grain boundary layer during thermal exposure, limiting the grain growth of plasma‐sprayed coatings. Consequent hot isostatic pressing (HIPing) of CNT‐reinforced Al2O3 at 1773 K shows differences in grain growth kinetics, degree of densification, and pore shrinkage. Densification of HIPed coatings is observed to be dictated by CNTs, phase transformation, initial grain size, and time of thermal processing. CNTs have shown to impede the Al2O3 grain growth by serving as grain pinning obstacles. Impediment of grain‐boundary mobility with variation of CNT content, and time and temperature of the heat treatment of aluminum oxide (Al2O3)–CNT nanocomposite is addressed in detail.