Twins in a Ti-50.0 at%Ni martensite were studied by ordinary electron microscopy and diffraction, and high resolution electron microscopy (HREM) to characterize the twins in the martensite and to observe the twin boundaries on an atomistic scale. ⟨011⟩ Type II twins were most profoundly observed and confirmed to be a lattice invariant shear. {\bar1\bar11} Type I twins, which were observed less frequently, are also proposed to be a lattice invariant shear from the following reasons, especially in polycrystalline specimens. Firstly, solutions exist for the phenomenological crystallographic theory for the twinning as a lattice invariant shear. Secondly, twins extend over the whole martensite plate with rather regular intervals. Thirdly, the observed twin width ratio agrees fairly well with the value predicted by the theory. A new twinning mode of (100) compound twinning was found, and the twinning mechanism was clarified.
HREM observations were made for ⟨011⟩ Type II twin, {\bar1\bar11} Type I twin and (001) compound twin. The {\bar1\bar11} Type I twin boundary was found to be sharp and coherent. Several observations were made to clarify the irrational natures of the ⟨011⟩ Type II twin boundary. However, the results were not conclusive, since the observation from the unique η₁ axis was not possible, which permits the observation of the twin boundary edge-on.