Herein, cellulose acetate nanofibers were manufactured by electrospinning technique and hydrolyzed by alkaline hydrolysis. Size effect of AgNPs was observed against E. coli. The structure and composition of nanofibers were characterized by scanning electron microscope (SEM), transmission electron microscope (TEM) and Fourier transform infrared spectroscopy (FTIR) and Thermal behavior was analyzed by thermogravimetric analyzer (TGA). Furthermore, AgNPs were incorporated on deacetylated nanofibers and then oxidized by KMnO4. AgNPs decorated cellulose nanofibers exhibiting strong bactericide activity against Escherichia coli BH5 alpha. Smaller in size 16.69 nm exhibited higher inhibition activity as compared to larger size 42.33 nm of AgNPs. It was shown that size of AgNPs has an effect on antimicrobial activity.