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 871°C where the alloy exhibited the lowest ductility. Dense dislocation network formed at γ/γ´ 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 760°C and at and above 871°C but it reduced stress rupture life of the alloy at 816°C γ´ coarsening and coherency loss at the γ/γ´ interface during thermal exposure were primarily responsible for the deterioration of mechanical properties and characteristic deformation behavior of the alloy.