Thin TiBx film was formed on Si (100) substrate by dual beam ion assisted deposition method using two Ar+ ion beams of the same energy at 15 keV and TiB2 target. The microstructure of the film was examined using transmission electron microscopy methods in as - deposited state and during in - situ post - deposition heat treatment conducted at 20 - 550°C temperature range. The crystallization of TiBx film starts at 550 °C and short isothermal heating at that temperature allows to obtain crystalline/amorphous nanocomposite. In the TiBx thin film nanocrystallites of TiB and TiB2 phases were identified. Post-deposition heat treatment induced also crystallization in thin amorphous subsurface layer of Si substrate, formed during deposition by intensive bombardment of assistant beam of Ar+ ions.