This article reports on the synthesis and full characterization of innovative silica-based nanoparticle composed of fumed silica as a core decorated with polyethylenimine (PEI) with different molecular weights (25, 10 and 1.8 kDa). Wide range of analytical, spectroscopic, and microscopic methods (TEM, DLS, ζ potential, elemental analysis (EA), TNBS and FTIR) were used to characterize the nanoparticles. Furthermore, transfection efficiency of these nanoparticles as non-viral vector was examined. The silica-PEI conjugates retained both the ability of PEI to fully condense plasmid DNA at low N/P ratios and suitable buffering capacity at the endosomal pH range. PEI-functionalized silica remarkably enhanced EGFP-N1 gene expression in murine neuroblastoma (Neuro-2A) cells up to 38 folds compared to PEI 25 kDa. Meanwhile the results of the cytotoxicity assays indicated that these silica-PEI conjugates have acceptable level of viability.