New experimental data is reported for CuO, MgO, TiO₂, ZrO₂ and Al₂O₃ water based nanofluids to enhance thermal conductivity by altering particle size. The effect of particle size alteration on thermal conductivity of nanofluids is verified for five different nanofluids, which is determined experimentally by hot-wire method. In the present work, probe sonication process is used to alter particle size of nanomaterial dispersed in base fluid. The probe sonication process is restricted to 60 min as further increase in probe sonication time does not show any significant decrease in particle size. As observed, all nanofluids show enhanced thermal conductivity than the base fluid. Among all nanofluids, Al₂O₃-water nanofluid (60 min) shows best thermal conductivity over base fluid (by 1.90 times).