Crystal structures of CVD diamond particles were observed in detail by transmission electron microscopy. The particles were deposited on silicon substrates under various conditions and had different levels of adhesion. An interlayer of SiC was found along the interface obtained with low methane concentration in the source gas, but it disappeared when deposited with higher methane concentrations. On the other hand, the size of crystal grains became smaller with increasing methane concentrations. From the correspondence between the structures and the adhesive toughness of these particles, it was suggested that the effect of both the SiC interlayer and the variation of grain size resulted in the complicated trend of adhesion with respect to methane concentration.