Abstract The martensite structure and substructure in a Ti36.5Ni48.5Hf15 high temperature shape memory alloy were investigated by X-ray diffraction, transmission electron microscopy (TEM) and high resolution electrom microscopy (HREM). The high temperature phase of this shape memory alloy was determined as being CsCl (B2) type structure with a = 0.309 nm. The martensite structure is B19′ type monoclinic structure with a = 0.293 nm, b = 0.411 nm, c = 0.473 nm, β = 100.4° and the space group is P21/m. The substructure in a martensite variant is (001)B19′ compound twins. The orientation relationship between B2 and B19′ was determined to be: B19′ ∥ B2 and (011)B19′ ∥ (001)B2, and the junction plane between two martensite variants was determined to be (011)B19′ ∥ (001)B2 or (1, − 1, 0.64)B19′ ∥ (1 − 1 0)B2.