One of the major limitations of effective nonviral gene carriers is their potential high cytotoxicity. Conjugation of polyethylene glycol (PEG) to polymers is a common approach to decrease toxicity and improve biodistribution. The aim of this study was to evaluate the effect of PEGylation on generation 5 polypropylenimine (PPI) dendrimer by using PEG moieties or alkyl-PEG groups. Polymers were synthesized by grafting of 5 and 10 % primary amines of PPI to NH2-PEG-COOH or Br-(CH2)9-CO-NH-PEG-COOH through Amide bond formation or nucleophilic substitution, respectively. Transfection efficiency and cytotoxicity were analyzed after 4 and 24 h exposure of neuroblastoma cell line (Neuro-2a) with synthesized vectors. Among all of the PEG-PPI derivatives, 5 % PEG-conjugated G5 PPI with alkyl chain (PPI-alkyl-PEG 5 %) resulted in the most efficient gene expression. This vector also significantly decreased the in vitro cytotoxicity and sub-G1 peak in flow cytometry histogram after 24 h incubation. Our results indicate that modification of 5 % primary amines of G5 PPI with PEG using alkyl chain as linker produces a promising vector combining low cytotoxicity, appropriate biodegradability, and high gene transfection efficiency.