Based on the ordinary electroless plating, electroless composite plating was developed as a new surface-treatment technology by adding high-hardness, high wear-resistant particles. Reserving the advantages of electroless coating, the composite plating evidently improves the wear-resistance of the coating. The technique of composite plating shows wide applicable prospect in the industries of machinery, electronics, spinning and chemical engineering, et al. In the present work, the deposition technology and its mechanism of composite particles, as well as the structure and the properties, of electroless Ni-P-Diamond composite plating were studied.Based upon the study on the deposition technology, the co-deposition mechanism of diamond particles in Ni-P-Diamond electroless composite plating was put forward. The structure of the composite coating was characterized systematically by analytical techniques such as OM , SEM+EDX, XRD, and DSC, and the influence of composite particles on the structure of Ni-P coating was discussed. The tribological properties of the coating was measured by M-200 abrasion testing machine, the mechanism of the improving wear-resistance of the composite coating was dissertated by abrasion and surface contour sketch analyzing, and the effective factors of abrasion was discussed. By three-point bending and rotating bending comparative fatigue tests, and by applying the automatic signal-collecting technology to track the dynamic variation process of the macro-deflection, combining with micro-morphology observing of break, the fatigue behavior of the specimens with Ni-P or Ni-P-Diamond coatings on 40Cr base was measured and analyzed. The main and valuable conclusions are as follows:1. By studying the co-deposition mechanism of diamond particles in Ni-P-Diamond electroless composite plating, it is found that the potential of diamond in the solution, , is negative, which means the diamond in electrolyte will not charge positive by adsorbing nickel ion, the adsorbing forcing bring about the strong adsorption is not the electrophoretic force, but the mechanical force. The calculation results show that when the deposition condition (except the amount of diamond added in the bath) is unchanged, the relationship between (l-av)cv/av and cv is linear, in which av is the diamond content in the coating, and cv is the diamond amount added in the bath.2. By studying the structure of the coating, it is concluded that:1) Under the technologic condition selected, the P content of Ni-P-Diamond composite coating is lower than that of Ni-P coating, which is about 10%(wt%). According to the results of TEM electronic diffraction, it is found that the structure of Ni-P-Diamond coating is not completely amorphous, containing some micro-crystal.2) Ni-P-Diamond coating starts to crystallize when heat-treated at 300C,and Ni, Ni3P phases are formed. Crystallization at 300 C is propitious to form Ni3P phase. As the isothermal time prolonged, the content of Ni3P in the coating is increased, but the content of Ni is decreased at 300C. The crystallization at 400 C is propitious to form Ni phase, the content of Ni3P in the coating is decreased, but the content of Ni is increased with prolonged isothermal time. The content of Ni phase in Ni-P-Diamond coating is higher than that in Ni-P coating under different crystallization heat-treatment. The TEM study on isothermal crystallization process of Ni-P-Diamond coating at 300 C shows that more and more crystal nucleus appeared as the isothermal time prolonged, the crystallization model is one-off crystallization.3) In crystallization dynamics studying, it is found that the crystallization activation energy of Ni-P coating is 310KJ/mol, and that of Ni-P-Diamond coating is 290KJ/mol. The crystallization onset temperature of composite coating is lower than that of Ni-P coating, but the peak temperature is higher than that of Ni-P coating under different heating-rates.