We investigated dielectric properties of perovskite nanosheets with different compositions (Ca2−xSrxNb3−yTayO10) to study the influence of A- and B-site modifications on dielectric properties. Langmuir–Blodgett deposition was employed to fabricate multilayer nanofilms of perovskite nanosheets. In these nanosheets, the A-site modification by Sr2+ ions causes the increase in εr value, whereas the B-site modification with Ta5+ ions improves leakage current characteristics and reduces permittivity value. The optimized property (εr = 240) is obtained for Sr2Nb3O10 nanosheet. We also discuss high-κ properties of Ca2−xSrxNb3−yTayO10 nanosheets by detailed investigations on first-principles calculations and interfacial structures.