The carbon nanotube (CNT) is a kind of cold cathode material with excellent field emission performance. It has a low threshold electric field and a high emission current density which make it attractive for the potential application in the flat panel display. The Fowler-Nordhiems theory and the mechanism of CNT field emission are clarified in this paper. The effects on field emission characteristics of CNT by different factors, such as the structure of CNT, the orientation, structure, density of the CNT arrays, adsorbed gas, work environment and so on, are discussed in detail. Result shows that CNT arrays can have a sufficient field enhancement and excellent field emission performance as which is of vertical alignment, high ratio of length to diamenter, moderate density and clean surface. (From Journal of Synthetic Crystals)