The co-precipitation hydrothermal method was used to prepare MWCNT (0.01g and 0.04g)/MnFe2O4 composites and the valence state elemental compositions were verified with XPS. The TEM micrograph revealed that the functionalized MWCNTs were well-dispersed in the matrix and retained their tubular structure. PL analysis indicates an increase in electron-hole pair separation, electron trapping in the MWCNT and an increase in structural defects. The result of positron annihilation lifetime spectroscopy (PALS) reveals that the concentration of composite surface defect increases with CNT. Additionally, these findings show that the vacancy clusters and voids are more prominent in the interface of the MWCNT/MnFe2O4 composite compared to the MnFe2O4 particle surface. N2 adsorption-desorption isotherms analysis confirmed the surface area change in the composite. Notably, the MWCNT/MnFe2O4 composite exhibited significantly enhanced photocatalytic activity compared to pure MnFe2O4, leading to superior efficiency in the removal of methylene blue (MB).