Three-dimensional stress and strain analysis considering the effect of strain rate on polyester under uniaxial tension and torsion was investigated by the scattered-light method. The difference in principal strains on the symmetrical section in tension problems could be calculated using two kinds of scattered fringe patterns obtained by two different incidences of polarized light. The shearing strains on the symmetrical section in torsion problems could be calculated using scattered fringe patterns obtained by one directional incidence. Finally, a new method was proposed for the estimation of the equivalent shearing stress and strain, the deviatoric stresses and strains on the symmetrical section in a three-dimensional model under uniaxial tension from the measurement of only the fringe gradient and its rate of increase. The method could estimate the equivalent shearing stress and strain in the model under torsion, too. This method was applied to a viscoelasto-plastic problem of a cylindrical specimen with a semi-circular notch under uniaxial tension and torsion. The distributions of strain rate, strain and stress on the notched section were estimated. The results were consistent with those calculated using the finite element method.