Electron field emission from polycrystalline diamond films has been investigated. Electron emission was measured locally at randomly chosen point on a diamond film fabricated by a microwave plasma chemical deposition method. In the original film, there were some points with a large emission current where flaws were found after the measurements, some points with a small and stable emission current without any flaw, and the other points with no emission. At the point of no emission, the film was electrically broken down by applying a high voltage. After the intentional breaking down, a small and stable emission always appeared there with no flaw. The maximum emission current extracted from an emission site was usually 1µA with no structural flaw found after the measurements. By using a simple model of emission site consisting of a core conductor embedded in insulator, the limitation of emission current is estimated from heating by the current and heat transfer to the insulator.