Ni-diamond composite coatings were prepared by co-electrodeposition. The effect of the particle content in suspension on the mass fraction of codeposited diamond was investigated. Codeposition process of Ni-diamond system was discussed according to two-step adsorption, particles transmission and interfacial force. The results show that the diamond contents in coatings increase firstly and then decrease slowly with the increasing of particles contents in baths. Co-deposition mechanism for Ni-diamond does not match well with N. Guglielmi’s two-step adsorption model and Yeh’s particles transmission model. Further analysis indicates that the codeposition process of Ni-diamond system could be divided into two periods. During the first period, particles in the baths are transferred to cathode surface by stirring. At the second period, particles are buried in matrix metal by the action of interfacial force with the help of high field intensity of electric double layer.