The influence of multi-walled carbon nanotubes (MWNTs) modified with 1, 6-hexamethylene diamine (HMD) and MWNTs milled by airflow on the mechanical properties of composites was investigated. The results of MWNTs, imaged by SEM, and measured by FT-IR and XPS spectra, confirmed that HMD was grafted on MWNTs. Experimental results show that the modified MWNTs have better dispersion in nanocomposites and better interfacial bondings structure between the MWNTs and epoxy matrix than those of milled MWNTs in the epoxy matrix. Comparing with the pure epoxy, the tensile strength, breaking elongation and the impact strength of composites increases, respectively, 79.7%, 160.4% and 188.2%, when the dosage of modified MWNTs in composites is 2%.