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The nickel-based superalloy Inconel 718 is treated with Coupled Ultrasonic and Electric Pulse Treatment (CUEPT),and the surface grain is refined from the average size of 9550.0 nm to 287.9,216.3,150.5,126.3,25.8 nm by different effective treatment currents,respectively.The ultra-precision turning experiments are carried out on the processed workpiece after CUEPT.The exper-imental results show that the average cutting force increases with the decrease of surface grain size.Moreover,a mathematical model that can describe the relationship between grain size and cutting force is established,and the calculated results match the experimental results well.The calculated results also indicate that the variation of cutting force caused by the same variation of grain size decreases as the degree of grain refinement increases.Finally,the influence mechanism of grain refinement on cutting force is analyzed.The improvement of stability of grain boundaries and the increase of number of grain boundaries cause the increase of cutting force after grain refine-ment.