The influence of temperature on shear modulus and internal friction in ultrafine-grained copper processed by equal channel angular pressing (ECAP) was investigated in the temperature range from 150 to 520 K. Acoustic measurements were performed on the inverted torsion pendulum at the frequencies of $18 and $45 Hz. An irreversible shear modulus increase and a concurrent decrease in sound attenuation were observed in the temperature region from $350 to 450 K on the first heating of specimens. The activation energy E % 1.05 eV and the attempt frequency m 0 % 10 10 s Γ1 of the irreversible relaxation process were determined using the measurements at different heating rates. The overall decrease in the shear modulus in ECAP-processed copper was shown to be made up by two components: a temperature-independent and a temperature-dependent ones. The latter is accompanied with an additional internal friction of the relaxation type, which is reversible up to $350 K. An estimate of the activation energy for this reversible relaxation process was obtained. Possible mechanisms responsible for the anomalous behavior of the shear modulus and the sound attenuation in ultrafinegrained copper are discussed.