A novel internally quaternized and surface-acetylated poly(amidoamine) generation four dendrimer (QPAMAM-NHAc) was synthesized and evaluated for intracellular delivery of siRNA. The proposed dendrimer as a nanocarrier possesses the following advantages: (1) modified neutral surface of the dendrimer for low cytotoxicity and enhanced cellular internalization; (2) existence of cationic charges inside the dendrimer (not on the outer surface) resulting in highly organized compact nanoparticles, which can potentially protect nucleic acids from degradation. The properties of this dendrimer were compared with PAMAM-NH 2 dendrimer, possessing surface charges, and with an internally quaternized charged and hydroxyl-terminated QPAMAM-OH dendrimer. Atomic force microscopy studies revealed that internally charged and surface neutral dendrimers, QPAMAM-OH and QPAMAM-NHAc, formed well-condensed, spherical particles (polyplexes) with siRNA, while PAMAM-NH 2 resulted in the formation of nanofibers. The modification of surface amine groups to amide significantly reduced cytotoxicity of dendrimers with QPAMAM-NHAc dendrimer showing the lowest toxicity. Confocal microscopy demonstrated enhanced cellular uptake and homogeneous intracellular distribution of siRNA delivered by the proposed QPAMAM-NHAc nanocarrier. The results clearly demonstrated distinct advantages of developed QPAMAM-NHAc/siRNA polyplexes over the existing nucleic acid dendrimeric carriers.