The birefringence property of Hard Elastic polypropylene(HEPP) fiber is different from that of usual PP fibet，because it will produce a lot of microvoids on the stretching，which is observed by SEM．Furthermore，the elasticity of HEPP fiber will be reduced in an infiltrating medium(e．g．silicon oil)，and be constant in a unfiltrating medium(e．g．water)．When the medium was infiltrated into the microvoids，the surface energy will be decreased，and the e-lasticity will be reduced also．Meanwhile it is the microvoids structure that results also in change of its birefringence properties，becaust of Form Birefringence (△form)．The birefrin-gence of HEPP fiber is measured on the stretching in the different medium by polarized mi-croscopy using compensator．It was found that there is a maximum extreme value of HEPP birefringence to different draw ratio with increasing the refringence of various infiltrating me-dia．In this case，the refringence of HEPP fiber iS equal to that of the medium (n=1.54)，and Form Birefringence is equal to gero．