In situ studies of the thermal stability and transformations in titanium oxide Ti2O3 with corundum structure by using magnetic susceptibility method in the temperature range from 300 to 1200 K revealed that the crystal size had a large effect on the magnetic susceptibility value, structure and transformations in Ti2O3. Analysis of the experimental data showed that Ti2O3 microcrystals and nanocrystals were weak paramagnets; the value of magnetic susceptibility of microcrystals in absolute magnitude was twice as large as that for nanocrystals. The structure of Ti2O3 microcrystals remains stable during long-term annealing in vacuum, the crystal structure remains trigonal, and only the degree of Ti2O3 crystallinity changes. Vacuum annealing of nanocrystals in the temperature range from 300 to 1200 K demonstrated that the system was not stable: intermediate oxide Ti9O10 and Magneli phases Ti4O7, Ti4O7, Ti7O19 and Ti3O5 are formed depending on the annealing temperature.