The dynamic mechanical behavior of molecularly oriented semicrystalline polyethylene terephthalate (PET) induced via the equal-channel angular extrusion (ECAE) process was investigated. Dynamic mechanical analyses in both torsional mode and bending mode were utilized. The results indicate that the ECAE-oriented PET has a higher dynamic storage modulus above the glass-transition temperature than that of the reference (control sample). The combined effect of molecular orientation and crystallinity is responsible for the changes in the primary and secondary relaxations of PET. Further analyses show that the shifting and broadening of the primary and secondary peak positions in oriented PET are mainly due to the amorphous-phase orientation because the crystallinity of PET decreases upon the ECAE processing A good correlation was found between the structural anisotropy and the dynamic mechanical properties.