Abstract: Nickel -base ferromagnetic alloy Ni -Mn -Ga was investigated for phase transformation temperature affected by magnetic field by measuring magnetization curve, temperature dependence of the resistance, and thermal analysis. The results show that magnetic field application can increase phase transformation temperature 1.5 K with distinctive behavioral affected by field. Meanwhile, thermodynamic theory of martensitic phase transformation is applied to study temperature dependence of the chemical free energy and non-chemical free energy on nickel-base ferromagnetic alloy; a mathematical model is built and agrees well with experimental result.