Both hcp-Ti and CaF₂-type TiH_x (x ∼ 1.5) with preferred orientations have been grown in as-deposited Ti flims on NaCl (001) substrates held at room temperature (RT). In order to clarify atomistic growth processes of TiN_y films due to the implantation of nitrogen ions (N₂⁺) with 62 keV into Ti films, changes of the crystallographic and electronic structures of Ti thin films by heating and by nitriding during N-implantation were investigated by in-situ transmission electron microscope (TEM) observations combined with electron energy loss spectroscopy (EELS), with the aid of molecular orbital calculations. H atoms which constituted TiH_x in the as-deposited Ti films were released with heating up, and were completely released at 350°C. The H-released unstable fcc-Ti sublattice was then epitaxially transformed into hcp-Ti. The loss peak observed by EELS for TiH_x shifted to the lower energy side with the decrease in the electron density in the hybridised valence band due to the release of H atoms from TiH_x during heating. On the other hand, the TiN_y was epitaxially formed by the N-implantation into the hcp-Ti, through the transformation of the hcp-Ti to fcc-Ti sublattice, partially inheriting the atomic arrangement of the hcp-Ti and accompanying the occupation of O-sites of the fcc-Ti by N atoms. The loss peak during N-implantation into the hcp-Ti films gradually shifted to the higher energy side with the increase in dose, due to the increase in the electron density in the hybridised valence band. Taking into account the bonding interaction of Ti sublattices with ligand H or N atoms, the transformation mechanisms between fcc-Ti sublattices and hcp-Ti sublattices due to the release of H atoms or due to the implantation of N atoms will be clarified. [DOI: 10.1380/ejssnt.2009.625]