Electric field of less than 5 V/μm is enough to extract electrons from diamond, whereas field of one to two orders of magnitude higher is needed to extract electrons from metal emitter tips. Despite such low-threshold field, the difficulty in clarification of electron emission mechanism is the factor preventing diamond from being used in a practical application. Quite a few numbers of possible mechanisms were proposed to better understand the origin and properties of the observed emission. Most of these mechanisms, however, were based on the conventional I-V (Emission current-Anode voltage) characteristics. Energy distribution of the field-emitted electrons is essential in direct clarification of the mechanism, In this study, combined XPS/UPS/FES system was used to characterize the electron emission mechanism of doped chemical vapor deposited (CVD) diamond. The results indicated successful observation of the origin of field-emitted electrons from doped CVD diamond comparison with natural diamond, used as a reference.