Highly crystallized TiO2 nanocrystallites (10-50 nm in size) with controlled phase structures (anatase and rutile) were synthesized using a one-step process. Flying TiC micrometer-size powder was oxidized in ArO2 and Ar-H-2-CO2 plasmas with various configurations used for injecting oxygen into the reaction system. The oxide nanocrystallites formed were characterized by X-ray diffractometry, Raman spectroscopy, and high-resolution transmission electron microscopy. The TiO2 nanocrystallites formed in the Ar-O-2 plasma consisted of isolated single-crystal particles of anatase with spherical shapes and rutile grains crystallized in a hexagonal morphology. The fraction of anatase increased greatly with the oxygen flow rate in the Ar-O-2 plasma. On the other hand, oxidation of the carbide in the Ar-H-2-CO2 plasma gave exclusively pure rutile nanocrystallites, regardless of the CO2 concentration. Injecting oxygen at the plasma tail greatly reduced the size of the nanocrystallites. These results suggest that both the size and the phase compositions of nanocrystal line TiO2 can be controlled using this thermal plasma oxidation process.