The structures of electrodeposited alloy of nickelphosphorus system were investigated by means of X-ray diffraction, method. It is shown that the structures of those alloy are a solid solution of phosphorus in nickel while the contents of phosphorus are less than 5.4 at%. When the contents of phosphorus increase up to more than 7.8 at%, the structures of alloys suffer a tremendous change, the diffraction patterns of these alloys may be interpreted by a model of random-layer (turbostratic) structures. The distances between neighbouring layers are 2.04? and the diameter of the layers are 6-8?. The thickness of stacking of layers is decreased as the contentes of phosphorus in alloy are increased. For examples, it changes from 124? for 7.8 at% P to 13? for 18 at% P. The diffraction pattern of the later is just the same as that of the typical amorphous alloys.Using the data of the ratio of intensities of {lll} or the data of the radial distribution function, the rootmean-square of static distortion can be calculated as (u2)1/2=0.17?. This value can be used to interpret why an arbitrary displacement between two neighbouring layers would be possible even though the distance between neighbouring layers is smaller than the diameter of an atom. Because of the small dimension of the layers and the large static distortion of the atom, it is easy to change the orientation of layer domain and no cr-ystallographic boundary region are necessary for the variation of orientation, so that the specimens are the continue distribution of atoms as a whole bat not an aggregation of micro-crystals.The β{lll} corresponding to the breadth of {lll} reflection at half-maximum intensity have been measured for some specimens. The results show that βlll/β222222/cosθlll, where θ is the Bragg angle, this means there would be some other factors (for example, the component fluctuation), to affect the breadth of the reflection line, besides the size factor.