Water-based nanofluids were prepared with multi walled carbon nanotubes(MWCNTs) of different lengths in concentrations of 0.1,0.25 and 0.5 vol%.To improve their dispersibility,pristine MWCNTs were functionalized and cut into small lengths by reflux in an oxidizing mixture of 3:1 sulfuric and nitric acids.The initial length of the carbon nanotubes(CNTs;10-15 μm) was reduced to 203,171 and 134 nm after1,2 and 4h of reflux,respectively.Surface modification and the reduced length of the CNTs,improved the stability of the nanofluids with no significant sedimentation observed after 80 days.Furthermore,the thermal conductivities of nanofluids prepared using refluxed CNTs,were higher than that of the pristine CNTs.The thermal conductivity also increased with the nanofluid temperature.The nanofluid prepared with 1 h refluxed CNTs had the highest thermal conductivity.The enhanced thermal conductivity and stability of the nanofluids was attributed to the decreased length of CNTs. Water-based nanofluids were prepared with multi walled carbon nanotubes (MWCNTs) of different lengths in concentrations of 0.1,0.25 and 0.5 vol%. To improve their dispersibility, pristine MWCNTs were functionalized and cut into small lengths by reflux in an oxidizing mixture of 3 : 1 sulfuric and nitric acids. The initial length of the carbon nanotubes (CNTs; 10-15 μm) was reduced to 203,171 and 134 nm after1,2 and 4h of reflux, respectively. Surface modification and the reduced length of the CNTs, improved the stability of the nanofluids with no significant sedimentation observed after 80 days. Morerther, the thermal conductivities of nanofluids prepared using refluxed CNTs, were higher than that of the pristine CNTs. The thermal conductivity also increased with the nanofluid temperature. The nanofluid prepared with 1 h refluxed CNTs had the highest thermal conductivity. The enhanced thermal conductivity and stability of the nanofluids was attributed to the decreased length of CNTs.