The deformation structure of B2 type TiNi compound around 573 K has been investigated by transmission electron microscopy (TEM). Serrations are seen in stress-strain curve, which corresponds to the formation of various planar defects with twin relation. The dominant planar defect found in the specimens showing serration is {114} compound twin. The other defects are in mirror symmetry with respect to {113}, {115}, {447} planes and so on. These defects are considered to be <110> symmetric tilt boundaries in bcc structure by ignoring the atomic arrangement of B2 structure and are characterized with Σ value based on coincide site lattice (CSL) model. For instance, the Σ value of {114}, {113}, {115} and {447} boundaries are Σ9, Σ11, Σ27 and Σ81, respectively. Numerous (114) defects initially form at grain boundary and grow into grain interior. Some of those deflect to (-1–14) defects. In such case, the {447} Σ81defect is always observed at the interface of (114) Σ9 and (-1–14) Σ9 defects. This fact indicates that the sigma combination rule of the CSL model can be applied to the triple junction of defects. Similarly, {7710} Σ99 boundary forms at the interface of {114} Σ9 and {113} Σ11 defects. It can be concluded that the ductility of B2 type TiNi compound around 573 K is attributable to the formation of various planar defects with large shear strain and the increment of independent slip system due to the formation of planar defects, and that the arrangement of some planar defects conforms to the sigma combination rule of CSL model.