Room-temperature tensile property and fracture mode of B2-type polycrystalline CoTi intermetallic compound was studied as functions of microstructure and environment. Using a hot-rolled CoTi sheet, various microstructures were prepared by different heat treatments and pre-straining. Tensile elongation as well as ultimate tensile strength (UTS) of specimens deformed in vacuum (i.e. intrinsic tensile elongation and strength) depended on microstructure. The hot-rolled and recovered specimens showed higher elongation and tensile strength than the fully-annealed and aged specimens. Also, the pre-strained specimen showed the improved elongation and UTS in comparison with the fully-annealed specimen. On the other hand, tensile elongation and UTS of the specimens deformed in air were lower than those of the specimens deformed in vacuum, irrespective of microstructure. The extent of embrittlement was lower in the hot-rolled, recovered and pre-strained specimens than in the fully-annealed and aged specimens. SEM observation for the hot-rolled and recovered specimens showed that the quasi-cleavage fracture mode was dominant irrespective of testing environment. However, the fully-annealed, aged and pre-strained specimens consisting of recrystallized grains exhibited quasi-cleavage fracture mode mixed with intergranular fracture mode. The results showed that not only the intrinsic room-temperature tensile properties but also the environmental embrittlement of CoTi intermetallic compound depended upon lattice defects (such as dislocations and vacancies) in addition to grain size and morphology. (C) 2003 Kluwer Academic Publishers.