Mono- and dialkylated polyethylenimines (PEI-1R and PEI-2R) were used for the facile synthesis of gold nanoplates with a preferential growth direction along the Au (111) plane. It was found that polymer hydrophobicity greatly influenced the nanoparticle morphology. PEI-1R in the acidic aqueous solution with a smaller degree of alkylation effectively adsorbed on the surface of gold nanoplates with the protonated ethylenimine groups rather than being aggregated in the bulk aqueous phase to form polymer aggregates as compared to the situation for PEI-2R. Loose alkylated PEI aggregates in acidic solution promote the formation of gold nanoplates by means of the anion-induced cation adsorption on certain crystallographic facets during the growth of gold particles. Without incorporating alkyl groups, however, the TEM image of the gold colloid solution with PEI showed only the formation of spherical gold nanoparticles by the same process. The morphology of gold nanoparticles was tuned not only by varying the degree of alkylation of PEI samples but also by the solvent type and pH value of the solution. By utilizing differently alkylated PEIs as reducing agents, this facile synthetic procedure can selectively result in the formation of gold nanoplates at room temperature without an extra inducing process.