Aluminum oxide, both in amorphous and crystalline forms, is a widely used inorganic ceramic material because of its chemical and structural properties. In this work, we synthesized amorphous aluminum oxide nanoparticles using a capacitively coupled nonthermal plasma utilizing trimethylaluminum and oxygen as precursors and studied their crystallization and phase transformation behavior through postsynthetic annealing. The use of two reactor geometries resulted in amorphous aluminum oxide nanoparticles with similar compositions but different sizes. Size tuning of these nanoparticles was achieved by varying the reactor pressure to produce amorphous aluminum oxide nanoparticles ranging from 6 to 22 nm. During postsynthetic annealing, powder samples of amorphous nanoparticles began to crystallize at 800 °C, forming crystalline θ and γ phase alumina. Their phase transformation behavior was found to be size-dependent in that powders of small 6 nm amorphous particles transformed to form phase-pure α-AlO at 1100 °C, while powders of large 11 nm particles remained in the θ and γ phases. This phenomenon is attributed to the fast rate of densification and neck formation in small amorphous aluminum oxide particles.