Abstract: The abnormal phenomenon in the high temperature tensile test of seamless pressure tube made of 410 MPa grade steel is shown as follows:the yield strength of the corresponding material increases first and then decreases, as the test temperature increases. At the same time, the tensile strength values show similar abnormalities with the change of test temperature, what's more, the maximum is at 150, 200℃, respectively. The changes of microstructure for seamless pressure tube were studied by various means. It was found that the experimental phenomena were the result of the combined effect of test temperature and grain size.