The present paper deals with the investigation of the mechanisms of TiO2 nanotubes formation on titanium surfaces during anodization process. The samples were made of pure Ti Grade-2 and Ti-6Al-4V alloy. They were grinded, etched with 0,5 wt. % HF acid and anodized. The anodization was done in electrolyte containing 0,5 wt. % HF acid using DC power supply with graphite electrode as cathode. The samples were investigated by SEM, EDAX and XRD analysis. The results show two different mechanisms of formation of TiO2 nanotubes on the surfaces of both materials. During the anodization process the oxide formations, obtained on the pure Ti surface after etching, are oxidized to nanorods; the area between them is also oxidized and connects them. This thin oxide layer grows in the metal depth while the nanorods are dissolved thus forming the porous sponge-like structure which is further transformed in tubular. While on the surface of Ti-6Al-4V alloy oxide nanonuclei originate which transform their shape from nanoseed to bowl-like with clearly pronounced bottom and walls, growing in tubular structures. The type of the material defines the surface morphology after etching. Thus obtained morphology influences on the processes running rate in different micro-regions determining origination of the titanium nanotubes on different stage as well as by different mechanism. The field-enhanced oxidation and field-enhanced dissolution are the main processes for formation of TiO2 nanotubes during anodization. In the regions with prevalent oxidation processes the TiO2 nanotubes are formed earlier while in the regions with dominant dissolution processes the TiO2 nanotubes are formed on the later stage.