We report a detailed study of thermal-induced phase transformation in TiO2 nanocrystals (NCs) via X-ray diffraction (XRD) and Raman scattering (RS) spectroscopy. Vertically aligned anatase TiO2(1 1 0) NCs were grown on the sapphire (SA)(1 0 0) substrate at 550 °C by metal organic chemical vapor deposition, using titanium-tetraisopropoxide (TTIP, Ti[OCH(CH3)2]4), as the source reagent. The effects of thermal annealing of TiO2 NCs in oxygen atmosphere between 600 and 1000 °C were investigated. XRD and RS spectra showed the onset of the phase transformation process from the as-grown anatase TiO2(1 1 0) NCs into rutile TiO2(0 0 1) at the annealing temperature of 800 °C. At annealing temperature higher than 900 °C, pure rutile phase of TiO2(0 0 1) NCs were formed and the crystalline quality of TiO2 NCs could be further improved upon higher annealing temperature.