High purity diamond film has been grown by refined Combustion Flame method with high oxygen proportion. Snapshots of time series of the diamond film forming were taken and a model was derived. By observing the period of seeding to coalescing of diamond crystals to form film, it is realized that the contacting area is small and the calculated value was less than 15%. It is also found that the base layer of the film was porous as a result of non-uniform crystal growth. To enhance adhesion of the film, the contacting area between the film and the substrate must be increased, and the porous base layer must be thin and dense as much as possible. To achieve that, very high and uniform nucleation is the key. Under this view, the nanocrystalline diamond film on smooth substrate is preferred than rough surface to have strong adhesion.