Yes / This paper investigates the coupled effect of functionalized multiwall carbon nanotubes (MWCNTs-COOH), nanomontmorillonite (NM), and sodium dodecyl benzene sulfonate (SDBS) anionic surfactant on compressive and flexural strengths of cement paste. The response surface methodology (RSM) was used to optimize the content of the two nanomaterials and surfactant, and to analyze the effect of their interactions on mechanical properties and microstructural characteristics of the paste. Test results indicate that the simultaneous use of NM and MWCNT can lead to 30% gain in compressive strength and 40% increase in flexural strength. Using analysis of variance, it was possible to predict the optimal weight percentage of nanomaterials. Atomic Force Microscope observations showed that the use of NM and MWCNT can reduce the surface roughness of cement paste and refine porosity, thus reducing the risk of cracking at the cement matrix and improving the homogeneity of the microstructure.