Abstract: 6061 aluminum alloy composites using AlCoCrFeNi_2.1 high entropy alloy as reinforced phases were prepared by friction stir processing. The effect of processing pass on the microstructural uniformity, interfacial metallurgical bonding and mechanical properties of composites were mainly investigated. The results indicated that microstructural uniformity and mechanical properties of AlCoCrFeNi_2.1/6061Al composites were improved obviously with the increase of friction stir processing passes. The interface between matrix and reinforcements was well bonded with a certain diffusion layer in the composites. With the increase of processing passes, the thickness of the diffusion layer increased. Compared with 6-pass friction stir processed aluminum alloys without reinforced phases, the adding of AlCoCrFeNi_2.1 particles can refine grain and enhance the tensile strength of composites. The tensile strength and elongation of composites increased markedly with the increase of processing passes. There are evident particles aggregation areas in the fracture surface of 2-pass and 4-pass samples, while the particles uniformly distribute and there are many dimples, presenting a ductile fracture mode on the fracture surface of 6-pass samples. The phenomena are attributed to three strengthening mechanisms of load transfer effect, dispersion strengthening and fine grain strengthening.