Anatase phase formation in TiO₂ films obtained by atomic layer deposition (ALD) is investigated. At growth temperature close to 200 °C, the anatase phase of TiO₂ originates from crystalline seeds formed in an amorphous layer. These seeds, formed in the initial stages of a film growth, allow an expansion of the anatase phase in the amorphous parts and their transformation. This expansion occurs either during a further growth process or during a post-deposition annealing at relatively low temperatures (160–220 °C). The process of a lateral expansion of the anatase phase within the amorphous one was found to be thermally activated with an activation energy of 1.5 eV.