The raw water (RW) samples collected from natural sources are subjected to water treatment process, including reverse osmosis (RO), and are packed in bottles as packaged drinking water (PDW). Raw water (21 samples) taken from deep wells of Chennai and Secunderabad which are used in the production of PDW, were analysed for ²³⁴U, ²³⁵U, ²³⁸U, ²²⁶Ra, ²²⁸Ra and ²¹⁰Pb activity concentrations. Activity Concentrations of ²³⁴U, ²³⁵U, ²³⁸U, ²²⁶Ra, ²²⁸Ra, ²¹⁰Pb and ²¹⁰Po in PDWwere also analysed. The mean activity concentrations of ²³⁴U, ²³⁵U, ²³⁸U, ²²⁶Ra, ²²⁸Ra and ²¹⁰Pb in RW at Chennai were 12.1, ≤1.3, 7.1, 2.6, 27.5, and 16.3 mBq/L respectively. The mean activity concentrations of ²³⁴U, ²³⁵U, ²³⁸U, ²²⁶Ra, ²²⁸Ra and ²¹⁰Pb in RWat Secunderabad were found to be 40.9, 1.7, 41.5 84.5, 100.1, and 17.0 mBq/L respectively. The mean concentrations of ²³⁴U, ²³⁵U, ²³⁸U, ²²⁶Ra, ²²⁸Ra, ²¹⁰Pb and ²¹⁰Po in PDWat Chennai were found to be ≤1.3, ≤1.3, ≤1.3, ≤0.2, ≤1.7, 28.0 and 1.2 mBq/L at Secunderabad were found to be ≤1.3, ≤1.3, 1.7, 4.3, 5.0 and 28.1 mBq/L. The study indicated a considerable reduction in the concentration of natural radionuclides due to water treatment. The reduction ratios of RW to PDW for ²³⁴U, ²³⁸U, ²²⁶Ra, ²²⁸Ra were 97, 96, 94 and 95%. In case of ²¹⁰Pb, the PDW showed higher concentration of ²¹⁰Pb than RW. This was due to its in growth from ²²²Rn which was not removed in the RO process.