Nanoparticles biosynthesis is a potential method for synthesizing of nanoparticles due to its simplicity and non-toxicity. In the present study, microorganisms were isolated from routh of poultry on nutrient agar medium. Resulted isolates were screenedfor their abilities to synthesize extracellular silver nanoparticles (AgNPs). Isolate producing the smallest AgNPswas identified by sequencing its 16S rDNA gene, and the results revealed it to be anew strain of Bacillussp., which was assigned anaccession number of KT982274. Factors affecting the nanoparticle size were optimized achieving the smallest AgNPsparticle size ~4nm bya 96h culture and incubating mixture of equal volumes of bacterial filtrate and silver nitrate solution, at 40 C, for 18 h, and pH of 7. Spherical AgNPs of 4nm were characterized by high resolution transmission electron microscopy (HR-TEM), and Fourier transform infrared spectroscopy (FT-IR) gave bands of proteins at 1442.2, and 1631.2 cm^-1corresponding to bands of I and II amide, respectively. Antimicrobial effect of the produced AgNPsagainst pathogenic yeast cells showed that the Minimum inhibitory concentration(MIC) after was 132 ppm, and 265 ppm for Microsporumaudouinii and Trichophytonviolaceum respectively after 3 h of exposure. After 6 h, MIC was 8 ppm, 66 ppm, and 33 ppm for Microsporumaudouinii, Trichophytonviolaceum, and Candida albicans, respectively.