Abstract: A simple synthesis of silver/montmorillonite/chitosan (Ag/MMT/Cts) Bionanocomposites (BNCs) with controllable sizes is presented. In this research, reduction process of silver cations by UV-irradiation was performed. AgNO3, MMT and chitosan were used as a silver precursor, solid support and stabilizer, respectively. The properties of Ag/MMT/Cts BNCs were studied as a function of UV-irradiation time. We found that UV-irradiation disintegrated the silver nanoparticles (Ag-NPs) into smaller size until a relatively stable size and size distribution were achieved. The average size, size distribution and crystalline structure of Ag-NPs were determined by Transmission Electron Microscopy (TEM) and Powder X-Ray Diffraction (PXRD). The antibacterial activity of Ag-NPs was investigated against Gram-negative bacterium (Escherichia coli (E. coli)) and Gram-positive bacteria (Staphylococcus aureus (S. aureus) and Methicillin-Resistant Staphylococcus Aureus (MRSA)) by disk diffusion method using Muller-Hinton Agar (MHA) at different sizes of Ag-NPs. All of synthesized Ag/MMT/Cts BNCs were found to have high antibacterial activity. These results showed that Ag/MMT/Cts BNCs can be useful in different biological research and biomedical applications such as surgical devices and drug delivery vehicles.