Microstructural evolution during high temperature exposure and its effects on tensile and stress rupture properties of the Ni-base superalloy B1900 have been studied. Tensile deformation of the as-cast specimen was concentrated in the localized slip bands in general. Stacking faults and deformation twins were observed in the as-cast tensile specimen tested at 871degreesC where the alloy exhibited the lowest ductility. Dense dislocation network formed at gamma/gamma' interface during thermal exposure caused homogenous deformation in the thermally exposed tensile specimen. Thermal exposure did not have significant effect on the stress rupture lives of the alloy at 760degreesC and at and above 871 degreesC, but it reduced stress rupture life of the alloy at 816degreesC. gamma' coarsening and coherency loss at the gamma/gamma' interface during thermal exposure were primarily responsible for the deterioration of mechanical properties and characteristic deformation behavior of the alloy.