For Co-Ni magnetic shape memory alloys, the microstructure and the micro-defects such as stacking faults should have important effect on the magnetically induced strain. Both the optical and TEM microstructure are shown in this paper. The stacking fault probabilities of Co-Ni alloys with different Ni contents were measured by X-ray diffraction method. It has been revealed that increasing of Ni content will decrease the stacking faults probability P-sf, resulting in the reducing of M, and a linear relationship between M, and the reciprocal of P-sf is established as M-s=654.6-0.567/P-sf. It has also been shown that the stacking fault probability decreases with increasing temperature, which can be expressed as: P-sf(x 10(3))= 1.726-1.626T.