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A simple, rapid and eco-friendly technique for the preparation of small-size silver nanoparticles with narrow distribution from 5 nm to 15 nm was developed. In the approach, using silver nitrate as silver precursor, and mulberry leaves extract as both capping and reducing agent, silver nanoparticles were obtained at room temperature, without employing any other reducing and capping agents. The formation of silver nanoparticles was observed by the change of color from pale yellow to brown and the UV-vis absorption spectroscopy. The effect of reaction time, temperature, silver nitrate concentration and mulberry leaves extract amount were studied. The silver nanoparticles were characterized by transmission electron microscopy (TEM), UV-visible spectroscopy (UV-vis), X-ray diffraction (XRD) and fourier-transform infrared spectroscopy (FT-IR). The results demonstrated that the obtained metallic nanoparticles were highly crystalline silver nanoparticles with a spherical shape and capped with extract. A possible formation mechanism was proposed. The method demonstrated in this paper may be extended to synthesize other noble metal nanoparticles by using renewable materials as capping and reducing agents.