By this work, we intended to understand the effect of ultrasonic sonication time on the structural and morphological properties of cerium oxide (ceria) nanoparticles. For comparison ceria nanoparticles synthesized by co-precipitation technique were considered. A series of controlled experiments were carried out to understand the crucial factors that influenced the formation of ceria nanostructures under a determined sonication time. The average crystallite size and the microstrain were obtained from W-H analysis of the X-ray diffraction (XRD) data. The variation in the bandgap values was discussed with respect to the crystallite size. For antibacterial studies, two known bacteria Escherichia coli (ATCC 25922) and Staphylococcus aureus (ATCC 25923) were used. On E coli, the prepared ceria showed bactericidal properties at 1000 μg ml-1, ceria obtained by sonication for 30 min was bactericidal at 2000 μg ml-1 and ceria made by sonication for 60 and 120 min were bactericidal at 3000 μg ml-1. While on S aureus, all the experimental conditions showed bacterial death at 3000 μg ml-1. The influence of the sonication time on the structural and antibacterial properties were discussed.