Abstract: Based on the multi-parameter mathematic model of theelastic stress field near a crack-tip, the influences of the stressintensity factors K_\rm I, K_\rm II, and T-stress on thephotoelastic isochromaticand isoclinic fringe patterns are presented for the mode I, mode II andmixed mode I/II crack, respectively. It is confirmed that T-stressinfluences the radius and rotate direction of the isochromatic fringe loops.The rotated angle of the loop only relates to the T-stress for the mode I orII crack, while to the intensity factors K_\rm I, K_\rm II, and T-stress for themixed mode I/II crack. There are two isotropic points along ±π/3directions in the mode I crack under positive T-stress, while none ofisotropic point under the negative T-stress. For the mode II crack only oneisotropic point appears behind the crack-tip under T-stress. For the mixedI/II crack, three isotropic points with different radius appear at thecorresponding positions under the positive T-stress, however, one pointexists behind the crack-tip for the negative T-stress. Thses isotropic pointsfor the mixed I/II crack comply with rules in the mode I and mode II,respectively.