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Titanium dioxide nanopowders were synthesised by the sol-gel method using TiCl 4 as a precursor under argon gas atmosphere. Effect of alteration of lattice strain with a change of synthesis parameters on the size of nanocrystallites and activation energy of phase transformation from anatase to rutile was investigated. Activation energy is the minimum energy required for overcoming the energy barrier for phase transformation between the two phases. The growth of nanocrystallite and a decrease in the positive lattice strain occurred when the calcinations increased from 300 • C to 550 • C. After the calcination increased at 600 • C, the sign of the lattice strain was altered from positive to negative, the growth of crystallite continued and phase transformation occurred. In spite of the fact that the anstase crystallites grew with the increase of the gelatinisation time, the activation energy decreased. In addition, the rapid decrease of the lattice strain in the as-prepared sample in lower mixing time causes an increase of the activation energy, and the mass fraction of the rutile phase occurred. IntroductionTitanium dioxide has been receiving substantial attention in the recent years due to its wide range of commercial applications, such as a photocatalysts, in white pigments, in gas sensors, in hygienic and medical instruments, in ceramics, and in vehicles external surfaces [1][2][3][4][5][6]. The applications of TiO 2 strongly depend on the high homogeneity and definite phase composition, particle size, high surface area and porosity. Moreover, in the evaluation of thermal stability and mechanical properties of nanocrystalline TiO 2 , grain size, surface free energy, activation energy of phase transformation and initial strain are very important factors [6][7][8][9][10][11][12][13][14][15][16].Titania naturally occurs in three polymorphs: anatase, rutile and brookite. Both anatase and brookite are metastable phases and rutile is a stable one [6,[17][18][19][20][21][22]. Each phase exhibits different physical and chemical properties, which are determined by their structures. Moreover, the theoretical study and experimental results have shown that the photocatalytic and photovoltaic properties of TiO 2 nanoparticles with two mixed polymorphic phases (anatase and rutile) are better than pure anatase TiO 2 nanoparticles [21][22][23]. One of the methods used to produce titania