Abstract: T91 steel is one of the candidate structural materials for the fourth generation reactor. The high temperature creep property after neutron irradiation is the key index to evaluate its service performance. In order to make full use of irradiation space, reduce irradiation parameter gradient and reduce specimen radioactivity, small-size specimens are needed for studying mechanical properties. However, the test data of small-size specimens may be different from that of standard specimens, which leads to the inaccuracy evaluation of material properties. In order to study the effect of specimen size change on the creep mechanical behavior of T91 steel, creep tests of standard rod specimens and small piece specimens of T91 steel in the temperature range of 675-725 ℃ and creep stress range of 80-120 MPa were carried out, the creep behavior and fracture mechanism of T91 steel between two specimens under the same test conditions were compared, the mechanism of influence of specimen size on creep properties was deduced, and the variation of size effect with creep temperature and stress and its reason were analyzed. Comparing and analyzing the creep test results of the two specimens, it is found that creep curves of two specimens of T91 steel under various test conditions consist of three stages—primary creep stage, steady-state creep stage and accelerated creep stage. With the increase of test temperature and creep stress, the steady-state creep stage shortens, the rupture time of T91 steel decreases and the steady creep rate increases. Compared with the standard rod specimen, the small piece specimen has longer creep rupture time and smaller steady-state creep rate. The creep activation energy of the two specimens is roughly the same, which means that the change of specimen size does not affect the creep deformation mechanism of T91 steel, while the reason for the difference is the difference of specimen stress state. Microanalysis results show that the fractures of all specimens are microporous aggregation ductile fracture, which means specimen size does not affect creep fracture type of T91 steel in microscopic scale. The dimple size of the small piece specimen fracture is relatively smaller, which means the poor creep fracture plasticity of small piece specimen. The dimples on the creep fracture of standard rod specimens are mainly equiaxed dimples, but the dimples on the creep fracture of small piece specimens are mainly equiaxed dimples, which means specimen size has effect on the macro fracture mode of T91 steel. Based on the creep test results, microanalysis results and relevant literature research, it can be found that specimen size affects creep properties of T91 steel by affecting the stress state of the specimen, different processes of microstructure evolution in the creep process is an important reason for the complex variation law of creep specimen size effect with the creep temperature and stress, and the effect of oxidation on size effect is not obvious.