This paper describes the study on mechanical properties at different test temperatures of SA-738 Gr.B plate in both delivery state and simulated post-welding heat treatment state. And the properties of the plate were characterized by tensile tests, Charpy impact tests and drop weight tests. We found that as temperature increasing, the strength of the SA-738 Gr.B plate decreased when temperature was below 200°C. While as temperature decreasing, the Charpy impact absorb energies decreased, that is, toughness of the plates became worse gradually. Through drop weight tests, we obtained the nil-ductility transition temperature for the plate. The tests data indicated the SA-738 Gr.B plate possessed good strength and toughness, which can meet the design requirements for the containment vessel and some other components of nuclear plant. The tests data also showed that post-welding heat treatment may influence both strength and toughness, so attention should be paid to simulated post-weld heat treatment when purchasing materials for plant construction. By optical microscope and scanning electron microscope, microscopic pictures have been obtained for the plate to help analyze the properties of this material, and the microstructure was the ensurance of the strength and toughness.