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In this study, the effect of the presence of MWCNTs on the surface tension of paraffin-based fluid was examined. Surface tension measurements of samples containing MWCNTs (0.005–5 wt%) were performed at temperatures of 25–70°C by SITA dynotester. Surface tension corresponds to the adhesive and cohesive forces resultant. Adding nanoparticles diminishes the cohesive force and simultaneously strengthens the adhesive force. Owing to the presence of MWCNTs, the effect of adhesion on the surface tension was superior to the effect of cohesion which consequently diminishes the surface tension. The loading nanoparticles efficacy on surface tension is weekend as the temperature increases. On the other hand, the presence of particles diminishes the sensitivity of surface tension to the temperature growth. In other words, temperature and mass fraction weaken each other's effects. The highest decrease in surface tension is reported to be 18.6%, which occurs at the lowest temperature as well as the maximum mass fraction. The prediction of MWCNTs/liquid paraffin surface tension behavior was performed by applying nonlinear regression and artificial neural network techniques, and statistical calculations revealed that the R-squared for the first and second techniques was calculated to be 0.99 and 0.998, respectively. Both techniques accurately predict nanofluid surface tension. • Suspending MWCNTs in liquid paraffin • Inspection of nanofluid stability using DLS and zeta potential tests • Surface tension measurements of samples by SITA dynotester Measuring surface tension • Presence of MWCNTs diminishes sensitivity of surface tension to temperature growth. • Maximum decrease in surface tension occurred at lowest temperature and highest mass fraction. [ABSTRACT FROM AUTHOR]