Gold nanoparticles were synthesized using brown seaweed (Sargassum crassifolium) extract and chloroauric acid solution. This is an easy, cheap and environment friendly synthesis method for the formation of gold nanoparticles. The gold nanoparticles with varying amount of seaweed extract was characterized using Ultraviolet-visible spectroscopy. Moreover, Transmission Electron Microscopy characterization was used to observe the shape and size of gold nanoparticles. Experimental results revealed that varying the amount of brown seaweed extract can control the optical absorption spectra of the produced gold nanoparticles. Greater amount of brown seaweed extract will exhibit peak in the lower wavelength while smaller amount of seaweed extract will exhibit peak in the higher wavelength. It is believed that the wavelength of free surface electrons resonance is related to the shift of absorption peak. TEM images revealed a more spherical and smaller particles as the amount of brown seaweed extract was increased. This simple green synthesis method of gold nanoparticles will give a cost effective route in the mass production of gold nanoparticles for biomedical applications.