Abstract: Dispersion of T(0,1)-T(0,4) and L(0,1)-L(0,4) mode guided waves in the elastic steel T91 pipe with a ratio of inner radius to thickness(RIRT) equal to 4 has been investigated on the basis of linear elastic hypothesis,when the material property is influenced by high temperature.The disperse relationship changing for foregoing modes,which is caused by material property variation,is studied.It has been found that the variation of material property caused by temperature changing has a stronger influence on propagating characteristics with the increase of frequency-thickness product.And for 1-3 order guided waves,including longitudinal and tangential modes,the variation of elastic modulus plays an important role in influencing disperse curves.