Abstract: The effect of grain size on 550 ~650 °C 360 〜165 MPa stress rupture property of 316H austenitic stainlees steel 28 mm plate is studued and its rupture mode is analyzed. At 550 °C and by 360 MPa or 335 MPa stress, the stress rupture life of the fine grain (44. 5 μm ) plate only reaches 48. 92% or 51.98% of the coarse grain (89. 6 μm) plate. Wedge crack on the grain boundaries led to the rupture. Tlie grain size has more influence on the stress rupture property, and higher stress makes the effect increase furtherly. However, at 650 °C and by 165 MPa, the stress rupture life of the fine grain (44.5 μm ) plate reaches 91.35% of the coarse grain (89.6 μm)plate. In this case, the grain size has a less influence on the stress rupture property, and microscopic damage occurring during stress-rupture is caused by the growth and coalescence of cavities on grain boundaries.