Theoretical and numerical treatments were devoted to the derivation of the threshold field for field electron emission from amorphous diamond thin films. Heavily doped n(++)-Si in the geometry of the tip was used as the cathode substrate. A three-step process involving internal emission, electron transport in the coating, and vacuum emission was employed to understand the emission. The derivation results predict that the potential barrier height at the Si-diamond interface is the main parameter that governs the threshold field for emission, which is consistent with the experimental phenomena that have been observed.