Abstract: Plant fiber reinforced composites are being applied to various fields of life. The interfacial incompatibility between hydrophilic reinforcement and hydrophobic matrix limits the mechanical properties of composites. In this paper, the kenaf fiber was modified by carboxylated carbon nanotubes (c-MWCNTs) to explore the interface improvement mechanism of kenaf fiber/epoxy resin composites. Firstly, the kenaf fiber was pretreated by water and NaOH. The effects of different pretreatment methods on the grafting of c-MWCNTs by kenaf fiber were investigated by observing the changes of fiber diameter, infrared spectrum and fiber bundle fracture strength. Then the kenaf fibers were modified with 0.5wt%, 1wt% and 3wt% c-MWCNTs, respectively. Interfacial shear strength (IFSS) of kenaf fiber/epoxy composites was investigated by single fiber pull-out test. The results show that the diameter variation, the removal of non-cellulose components and the decrease of fiber bundle fracture strength of kenaf fiber after alkali treatment are the smallest and the dimensional stability of composite is higher. Through single fiber pull-out test, IFSS of the kenaf fiber/epoxy resin composite increases gradually, but the effectiveness decreases gradually. When the mass fraction of c-MWCNTs is 0.5wt%, the effectiveness is the highest, and the improvement effect reaches 45.09%. The interfacial properties of kenaf fiber/epoxy resin composites modified by c-MWCNTs are improved. Due to the existence of c-MWCNTs, the mechanical lock between fiber and resin matrix is strengthened.