The internal frictionQ

-1

of Cu（99.98%Cu） in the process of plastic deformation was measured by a middle torsion pendulum on a modified tensile testing machine. The effects of strain rate (in the range of 1.08×10

-6

/sec to 37.6× 10

-6

/sec) and frequency （in the range of 0.49 Hz to 4.16Hz）on Q

-1

were studied. The experimental data showed/that Q

?

increases with an increasing of ε（when f is constani） and/or ω

-1

（when ε is constant）

but the dependence of Q

-1

on ε or ω

-1

is not linear. The results of the data analysis indicated that the Q

-1

can be expressed as a sum of A（ε/ω） and B（ε/ω）

1/2

. The effect of the variation of dislocation average velocity with ε on the Q

-1

was discussed and the expression of average velocity of dislocation was obtained as function of ε.