The characteristics of premartensitic transition on nearly stoichiometric Ni 505Mn245Ga25 single crystals have been investigated systematic ally by various methods, such as the ac magnetic susceptibility, resistance, mag netostriction, premartensitic transition strain and magnetization measurement. T he premartensitic transition strain in free samples was reported, and the enhanc ed premartensitic transition strain with an external magnetic field applied alon g the different directions was investigated. A large magnetostriction up to 505ppm in the [001] direction of the parent phase is obtained in a pulse magnetic field of about 80kA/m applied along the [010] direction at the premartensitic t ransition point, which is over five times larger than that in the parent phase. Based on the soft mode condensation and characteristic of the crystal growth, th e mechanism of the premartensitic transition strain was analyzed. The effect of magnetic field intervened premartensitic transition strain is attributed to the competition between the crystal lattice deformation induced by the enhancing mag netoelastic coupling and the intrinsic deformation originated from the soft-mode condensation. We found that the premartensitic transition further generated the anisotropy between two crystallographic directions of [001] and [010], which is proved by the magnetization measurement. Furthermore, the resulting strain prop erties, such as the largest magnetostriction, the saturated premartensitic trans ition strain and saturated field, have been discussed in detail.