The martensitic phase transformation temperature,Tm and the Curie temperature, TC, of polycrystalline samples Ni2+xMn1-x Ga(x=-0.1,0,0.08,0.13,0.18,0.2) and Ni2-xMn1+x/2Ga1+x/2(x=-0.1,0,0.04,0.06,0.1) have been investigated by measuring the temperature dependence of alternating-current susceptibility. It was found that, with the increase of Ni content,TC decreases and Tm increases for Ni2+xMn1-xGa, and both TC and Tm increase first and decrease afterwards for Ni2-xMn1+x/2Ga1+x/2. Stress-free and two-way thermoelastic shape memory, with 1.2% strain, has been found in single crystal Ni52Mn24Ga24. The deformation can be enhanced more than three times, up to 4.0% shrinkage with a bias field 1.2T applied along the measurement direction. The origin of large recoverable strain should be attributed to the low level of internal stress and the field-induced rearrangement of martensitic variants through twin boundary motion rather than the phase boundary motion.