To improve the stability of a head tail type polycation (PAMAM dendron-PLL), which is composed of a polyamidoamine (PAMAM) dendron head and poly(L-lysine) (PLL) tail blocks, as a gene vector against serum proteins, the PEGylation of the PAMAM dendron head was examined using physicochemical and transfection experiments. The PEGylation of the PAMAM dendron-PLL polyplexes neutralized the zeta-potential, indicating that the PEG brushes surround the surface of the polyplexes. The PEGylated PAMAM dendron-PLL polyplexes had higher tolerance against incubation with serum proteins when compared with non-PEGylated PAMAM dendron-PLL polyplexes. In addition, the transfection efficiency of the PEGylated PAMA:M dendron-PLL polyplexes increased with increasing incubation periods, leading to an increase in the amount of pDNA uptaked by He La cells. Furthermore, effective stabilization of the PAMAM dendron-PLL polyplexes through PEGylation of the PAMAM dendron block was confirmed by the transfection experiments after preincubation with serum proteins and the PEGylated PAMAM dendron-PLL polyplexes showed a comparable level of transfection efficiency with polyethyleneimine polyplexes.