Abstract: The diamond films deposited on polished molybdenum by hot filament chemical vapor deposition (HFCVD) were investigated by scanning electron microscopy (SEM) and Raman spectroscopy. The results show that the films are composed of diamond crystallites, the crystal boundaries are mainly graphite and there are a number of defects in the films. The characteristic measurements of field emission indicate that the diamond film formed by high density of CH₄ exhibits lower threshold electric field required for emission than the diamond film deposited by low density of CH₄. It means that the impurity (such as graphite) and defects (dangling bonds) in the films greatly affect their field emission. Based on the results, a field emission mechanism of CVD diamond films is put forward, i.e. the dangling bonds in the films which enhance the electrical field in the films and the graphite in the films which increase tunneling coefficient of electrons can improve the field emission of CVD diamond films.