Based on the colloid aspects and the model of the structure transformations in diamond synthesis under the action of a solvent-catalyst, it is suggested that the coalescence of the puckered dimers of diamond as crystallized units is the main way of diamond nucleation. From experiments, a curve representing the qualitative relationship between the number of diamond nuclei and the temperature as well as the relationship between the diamond nucleation and the pressure are obtained. The roles of the above mentioned nucleation mechanism on directing the practice of diamond synthesis are discussed.