Abstract: The 15% and 25% expansion tests are conducted to the hot-extruded 10Mn25Si3A13 hi-manganese seamless steel tube,based on which the pro-test and post-test tensile property,impact energy and the microstructure of the tube body are compared.As a result the facts as follows are revealed. The structure of the hot-extruded hi-manganese seamless steel tube is of homogeneous fine austenite;the yield is 340 MPa;the yield ratio is 0.47;the elongation is 64%;the hardening index is 0.46;the half-size impact energy is 78 J,and the wall thickness deviation is less than 5%. All the above mentioned data mean that the tube has good uniform deformation ability,and the low yield strength is helpful for expansion. Due to the 15% and 25% expansion deformations,the yield strength of the tube is increased by 67% and 85% respectively,and the tensile strength increased by 19% and 27% respectively,whereas the elongation is reduced by 34% and 29% respectively,and the impact energy is decreased to 47 J and 38 J respectively. The post-expansion test mechanical properties of the tube are satisfactory to the applicable requirements for the N80 Grade steel as specified under API Spec 5CT.