Deposition of zirconium diboride films on Zircaloy-it substrates at 733 K over 20 cm is carried out by remote plasma enhanced chemical vapor deposition (RPECVD). Different post-discharge compositions (Ar-H-2, Ar-H-2-BCl3 and Ar-BCl3) are tested in several process configurations. Experiments performed by thermal CVD and RPECVD with Ar-H-2 post-discharge show that the deposition of ZrB2 films on oxidized Zircaloy-4 is impossible at temperature lower than 853 K. Ar-H-2-BCl3 post-discharges do not give to adherent films on oxidized Zircaloy-4 at a temperature lower than 753 K. It is shown that ZrB2 thin films can be synthesized by using flowing Ar-BCl3 microwave post discharges. Chlorine must etch the zirconia protective layer before zirconium diboride is synthesized. Therefore, the control of thickness of this zirconia layer by a previous oxidation treatment gives homogeneous deposition. The structure of the films has been determined to be nanograins of ZrB2 dispersed in an amorphous solid solution of boron and zirconium oxides. The origin of the boron species incorporated in the ZrB2 films is attributed to the eching of the quartz tube by chlorine.