Nanofluids is widely used in heat transfer application due to its good thermal conductivity. In steel heat treatment industry, nanofluid is common to be utilized as quenching medium. The advantage of using nanofluid as quenchant, the thermal conductivity can be controlled by varying the amount of the solid particle. Carbon nanotube (CNT) is one of the solid particles commonly used as nanoparticle in nanofluid. In this study, multi-walled carbon nanotube (MWCNT) was used with amount variation of 0.01%, 0.03%, and 0.05%. Additionally, sodium dodecyl benzene sulfonate was added from 10%, 20% to 30% as surfactant to stabilize the nanofluid. The effect of CNT and SDBS in nanofluid was observed in two areas i.e. thermal conductivity and zeta potential. From the result, it can be seen that CNT addition in base fluid did not significantly change the thermal conductivity. The highest measured thermal conductivity in 0.05% CNT was 0.794 W/m°C. This result was achieved by adding 10% of SDBS. Increasing amount of SDBS seem to decrease the thermal conductivity. However, SDBS addition increased the zeta potential value up to -60 mV, which means the nanofluid had a very good stability.