•Stable MWNTs and graphene nanofluids were used in a mechanical wet cooling tower.•Thermal and rheological properties of nanofluids were investigated.•Nanofluids enhanced the efficiency, cooling range and tower characteristic.•Water consumption reduced significantly for both MWNTs and graphene nanofluids. This study deals with an experimental investigation on the thermal performance of a mechanical wet cooling tower with counter flow arrangement by using multi-walled carbon nanotubes (MWNTs) and nanoporous graphene nanofluids. Stable nanofluids were prepared through two-step procedure by using water with properties taken from a working cooling tower in the South of Iran. Zeta potential revealed suitable stability of MWNTs and nanoporous graphene nanofluids. Thermal and rheological properties of the nanofluids were investigated. It was found that thermal conductivity increases by 20% and 16% at 45°C for MWNTs and nanoporous graphene nanofluids, respectively. The increase in density and viscosity, particularly in low concentrations of nanoparticles, was insignificant enough for industrial applications. Moreover, it was found that by using nanofluids, efficiency, cooling range and tower characteristic (KaV/L) are enhanced in comparison to water. For instance, at inlet water temperature of 45°C and water/air (L/G) flow ratio of 1.37, the cooling range increases by 40% and 67% for MWNTs and nanoporous graphene nanofluids (0.1wt.%), respectively. On the other hand water consumption is reduces by 10% and 19% at inlet water temperature of 45°C for MWNTs and nanoporous graphene nanofluids, respectively.