Ultrasonic devulcanization of tire rubber particles of 10 and 30 meshes by means of a new ultrasonic twin-screw extruder was investigated. The ultrasonic amplitude and devulcanization temperature were varied at a fixed frequency of 40 kHz. The die pressure and ultrasonic power consumption during devulcanization were recorded. The degree of devulcanization was investigated by measuring the cross-link density, gel fraction, and revulcanization behavior. Rubber of 30 mesh exhibited a lower die pressure and higher degree of devulcanization than that of rubber of 10 mesh. Because of the higher level of devulcanization and lower viscosity of devulcanized rubbers at higher amplitudes, the temperature of devulcanized rubbers at the die was reduced with an increase of the ultrasonic amplitude. Generally, the torque at the start of curing and maximum torque on the curing curve measured during revulcanization is higher for rubber of 10 mesh and rubbers devulcanized at a lower temperature. Rheological properties of devulcanized and revulcanized rubbers and mechanical properties of revulcanizates were measured. The complex viscosity of devulcanized and revulcanized rubbers of both meshes as a function of frequency exhibited a power-law behavior, with the power-law index being 0.06 for devulcanized rubbers and 0.02 for revulcanized rubbers. The consistency index of devulcanized and revulcanized rubbers was higher for rubber of 10 mesh; however, both rubbers showed a higher consistency index at a lower temperature. Cole–Cole plots, cross-link density, and gel fraction of devulcanized and revulcanized rubbers, revulcanization behavior, and modulus of revulcanizates separated in two distinct groups based on the level of devulcanization and effect on molecular structure of devulcanized rubber. Revulcanizates with a greater degree of devulcanization exhibited a higher elongation at break, whereas those with a lower degree of devulcanization exhibited higher strength and modulus. Revulcanizates of rubber of 30 mesh exhibited a consistently higher elongation at break. The normalized gel fraction versus normalized cross-link density was described by a unique function independent of the processing conditions and rubber particle size.