Carbon Nanotubes (CNTs) are functionalized with ethylenediamine by chemical cut and grafting process. CNTs/nylon-6 (PA6) composite is prepared by solution-evaporation method. The dispersivity of CNTs and mechanical properties of CNTs/PA6 composites are tested. Reinforced mechanism is also investigated. The results show that the ethylenediamine-functioned CNTs (s-CNTs) are well dispersed in the formic acid and the weight content of grafted ethylenediamine is about 30%. With only 2wt% addition of s-CNTs, the modulus, strength and strain of PA6 are dramatically improved by about 46%, 85%, and 200% respectively. The tensile course of s-CNTs/ PA6 composites shows that the increase of modulus were provided by both the increase of matrix crystallinity induced by s-CNTs and the recruitment of s-CNTs, and the improvements of strength and strain were attributed to the malleable integration between the s-CNTs and matrix, which was formed with s-CNTs, crystal matrix based on the tubes and the partial entanglements of tubes.