Plant fiber reinforced polymeric composites are increasingly applied in engineering applications, while the incompatible interface between the hydrophilic cellulose fibers and the hydrophobic polymer matrix remains a bottleneck for obtaining high mechanical performances. In this study, carboxyl-functionalized carbon nanotubes (COOH-CNTs) were successfully coated onto flax fibers using a "soaking or spraying-drying" process by taking advantage of the unique chemical composition of plant fibers. Single yarn tensile, single yarn pull-out, double cantilever beam, short beam shear, and drop-weight impact tests were performed to assess the effects of CNT coating on the properties of flax fiber reinforced composites. The maximum enhancements for interfacial shear strength (IFSS), mode I interlaminar fracture toughness, and interlaminar shear strength (ILSS) were 26, 31, and 20 %, respectively. Though the impact strength was kept unchanged, a maximum of 10 % reduction in the impact damage area was obtained due to the presence of CNTs. Fourier transform infrared (FTIR) suggested that hydrogen bonds between the hydroxyl groups of flax fiber and carboxyl groups of CNTs were formed which could strongly bind CNTs to the fibers. Microscopic analysis also showed the insertion of CNTs into the fibers, further strengthening the interaction between plant fiber, CNTs, and polymer matrix by interlocking. The multi-scale microstructures of flax fibers induced new mechanisms for enhancing the mechanical properties of flax fiber reinforced composites.