The preparation of metal nanoparticles is a major research area in technical engineering due to their unusual properties, such as catalytic activity, novel electronic, optic and magnetic properties and biotechnology. Specially, silver has been used for years in the medical field for antimicrobial applications because it known for its antimicrobial properties and even has shown to prevent HIV binding to host cells. Common synthesis, chemical and physical methods using chemical reducing agent and organic solvent are not too suitable to have application to bioengineering because they should have associated environmental toxicity or biological hazards. Development of sustainable processes through green chemistry is attractive about the elimination or minimization of chemical waste. Here, we introduce the green method for preparation of silver nanoparticles using chitosan oligomer as both reducing and stabilizing agent in water. We expect that the use of environmentally benign solvent and chitosan oligomer to prepare silver nanoparticles offers numerous benefits and compatibility for pharmaceutical and biomedical applications.