Abstract: The effects of reaction medium on the morphologies and sizes of the synthesized polydopamine nanoparticles (PDA NPs) were reported. The PDA NPs were prepared in different reaction medias including NaOH solution, Tris buffer solution, ammonia solution and phosphate buffer solution, respectively. The obtained PDA NPs were characterized by Fourier Transform Infrared (FF-IR) spectroscopy, Dynamic Light Scattering(DLS), Ultraviolet (UV-vis) spectrophotometry, Transmission Electron Microscopy (TEM). The effects of reaction medium on the morphologies and sizes of the synthesized PAD NPs were ivestigated in detail. The experimental results showed PDA NPs were successfully synthesized in these reaction media at pH 8.5. The synthesized NPs were dispersed in the reaction media uniformly.