Abstract: In order to prepare composites with fine grains, uniform microstructure, and improving the mechanics performance significantly, different contents of carbon nanotubes (CNTs) reinforced aluminum matrix composites were prepared by friction stir processing (FSP). The microstructure, tensile properties and morphologies of tensile fracture surface were analyzed. The results show that CNTs are embedded in the aluminum matrix. The grain size is fine in the center of the friction stir zone. CNTs and matrix have a good combination. There are no obvious defects in the composites. CNTs improve the mechanics performance of the matrix. The tensile strength of aluminum matrix increases with the increasing of CNTs contents. The tensile strength reaches to 201 MPa when the volume fraction of CNTs is 7%, which is 2.2 times than that of matrix. The composites show the characteristics of brittle fracture at the macrostructure and plastic fracture at the microstructure. The fracture mechanism dominates by CNTs/Al interface debonding, matrix tearing and reinforced fiber fracturing.