Carbon nanotubes (CNTs) are excellent field emission cold cathode materials. They have a low threshold electric field and a high emission current density which make them attractive for potential application in flat panel display. In this paper, the Fowler-Nordheims theory and other mechanisms of CNTs field emission were clarified; The effects on field emission characteristics of CNTs by different factors, such as the structure of CNTs (MWNT $versus$ SWNT), the orientation, geometrical structure, density of the CNTs arrays, the interaction of CNTs with the supporting suface and the environmental condition and so on, were discussed in detail; The practical applications of field emission characteristics from CNTs in flat panel display areas were also introduced.