The martensitic transformation in Co50Ni20Ga30 ribbon has been characterized in detail by means of X-ray diffraction and magnetic measurements. The sample showed a structural transition from the body-centered cubic with indexed lattice parameters of a=5.743 Å to martensite with a tetragonal structure with lattice parameters of a=b=5.422 Å and c=6.401 Å. (c/a>1). By comparing the results of the diffraction intensity and the magnetic susceptibility measurements, we found that the proceeding of the martensitic transformation can be divided into several different steps during cooling from 283 K to 213 K. During heating from 673 K to 833 K, the peak width became very wide and the intensity became very low. It reveals an order-disorder transformation.