Abstract: Aim\ To study the chemical and physical processes which ultrafine diamond (UFD) experienced during its formation Methods To establish the physical and chemical models on the basis of experimental analysis, and to prove the possible liquid carbon from basic thermodynamic relationship and related data Results The chemical and physical pictures of the synthesis process were proposed and the relationship between UFD size and the melting point under the detonation thermodynamic condition was derived Conclusion As viewed from physics, the formation of UFD is a consecutive (fast and slow) process; as viewed from chemistry, it is also a consecutive (detonation and oxidization, graphitization) process Due to the relationship between size and melting point of ultrafine particles, the UFD experiences liquid state during its formation