Nucleation of diamond is of great importance for its growth, and a detailed understanding of the nucleation process is, therefore, desired for many applications. The pretreatment of the substrate surface may impact the initial growth period. This study demonstrates the synthesis of diamond films by microwave plasma chemical vapor deposition on a Pt/SiO2/Si substrate. The Pt particles were deposited on the SiO2/Si surface at room temperature, whereas adamantane was seeded on the SiO2/Si surface by ultrasonic treatment. The Pt particles on the SiO2 surface behave as a catalyst, which adsorb hydrocarbons from adamantane contained plasma and give distinct features of carbon in the early stage deposition. The adamantane transforms not only in the nanodiamond phase but also in other carbon phases, which can then act as nuclei for diamond growth. These kinds of phases enhanced the diamond density at partially low temperature deposition. The presence of an oxide intermediate layer between Pt and Si prevents silicidation as well as SiC in diamond deposition.