Due to small size, large surface area and unique physiochemical properties of the nanoparticles enhances their interaction with the microbes and combined with the growth inhibitory capacity against microbes to carry out a broad range of probable antimicrobial activities. The antibacterial and antifungal activities of nanoparticles in the biomedical field has been developing very vigorously now days. For this, inorganic compounds have a great of interest due to its stability under harsh processing conditions and stabilization compared with organic antibacterial agent. Authors have chosen CdS nanoparticles, it can easily interact with other particles to increase their antibacterial efficiency because they possess extraordinary photocatalytic ability for the photo-degradation of di erent dyes under the irradiation of UV light. In this book we report antibacterial activity of Co doped CdS nanoparticles with different molar concentrations on Gram-positive and Gram-negative bacteria.