Abstract: Based on Agarwal's approximate strain-optic law,a new approximate strain-optic law for photoelestic composite mat; rials is proposed in this paper.The principal strain linear difference and the direction of major principal strain can be obtained from only two photoclastic measurements(isochromatic-fringe order and isoclinic angle) by means of the strain-optic law.Thus,the photoclastic analysis of composite materials becomes as easy as that of isotropic materials.Analyses of accuracy and verifications are made for the strain-optic law.The obtained result gives a smaller error than that from Agararwal's approximate strain-optic law when the principal-strain directions are near the principal directions of materials.The accuracy is significantly improved for the approximate analysis of photoelastic composite materials.Consequently,the new law provides the theoretical basis for the application of photoelastic analysis of composite materials.