Neutron diffraction technique is used to identify the crystal structures and study the phase transformation in a ternary Ni55Fe20Al25 alloy, prepared in the ordered and site-disordered states by annealing and rapid-melt-quenching techniques. We report the observation of ‘step-like’ wavy structures in the thermomagnetic curves taken on ordered samples. No such features are observed in the site-disordered counterparts. Negative magnetoresistance at 10 K and in a magnetic field of 80 kOe in the disordered sample is nearly two times larger than that in the annealed sample. A tentative explanation for these phenomena is given.