<p>The effects of ultrasonic amplitude on cutting forces and machined surfaces were investigated through conventional milling and longitudinal-torsional ultrasonic vibration milling tests of TC18 alloy. The cutting force, surface morphology, surface roughness, residual stress, and sub-surface microstructure during longitudinal-torsional ultrasonic vibration milling under conventional milling and different ultrasonic amplitude conditions were studied in detail. The experimental results show that longitudinal-torsional ultrasonic vibration milling can reduce the cutting force and surface roughness, increase the residual compressive stress on the machined surface of the specimen, and form a certain depth of plastic deformation layer with direction under the machined surface. As the ultrasonic amplitude increases, the average cutting force and surface roughness in longitudinal-torsional ultrasonic vibration milling show a decreasing trend, while the surface residual compressive stress shows an increasing trend. Additionally, the thickness of the plastic deformation layer and the degree of grain refinement increases with the increase of ultrasonic amplitude. Compared to conventional milling, the average radial cutting force of longitudinal-torsional ultrasonic vibration milling can be reduced by 17.08%, the surface roughness can be reduced by 28.47%, the surface residual compressive stress can increase by 9.7%, and the depth of the subsurface plastic deformation zone can increase by 27.66%.</p>