In order to synthesize alternative drugs from the natural sources this work was carried out to synthesize the silver nanoparticles from the haemolymph of two marine crabs, Carcinus maenas and Ocypode quadrata and to determine their influence on human and fish pathogens. The silver nano particles were synthesized from haemolymph and analysed by UV spectroscopy and SEM. The antimicrobial activity of haemolymph and silver nanoparticles from haemolymph were tested against human and fish pathogens. Our recent world approaching an increasing commercial demand in nanoparticles (NPs) due to their broad applicability in various field such as catalysis, chemistry, electronics, energy, and medicine. These NPs can be synthesized by various physical, chemical and biological techniques for the development of NPs. In our research, we successfully synthesized the NPs from the haemolymph of Carcinus maenas and Ocypode quadrata. Ag synthesis was confirmed by the colour change, wave range of UV-Vis Spectroscopy and the shape and size of the NPs determined by the SEM. It shows moderate and high toxic to the human pathogens as well as the fish pathogens. UV–VIS spectrum showed absorption peak at around 380 and 420 nm. The synthesized silver nanoparticles were clustered and the size ranged from 45-50 nm. The present process showed that haemolymph is an excellent biomaterial for the synthesis of Ag NPs with antimicrobial activity for nanotechnology based industries.