The fs-laser induced modification of the refractive index was investigated for pure fused silica (Corning 7980) under X-ray preconditioning (20-100 keV). Three samples have been pre-irradiated with different accumulated doses of 0.5 MGy (SiO2), 1 MGy, 6 MGy and one kept as a reference. Waveguides have been inscribed in all these samples varying the pulse energy and scan speed. The X-ray treatment influence on the refractive index change (∆n) was investigated through the estimation of the numerical aperture (NA) of the written waveguides. Results show that ∆n is higher in the pre-irradiated samples with Xrays compared with the untreated one. As a consequence, the photoinscription efficiency increases and the pretreatment can be considered as an option for high contrast low loss index modifications.