Aligned carbon nanotube (CNRT) films exhibit excellent electron emission properties at very low fields. This has been attributed to the high aspect ratio of CNTs, I.e. Their emitting surface can have high local field due to geometrical enhancement. It is found that a new mechanism can be responsible for the enhancement of the local field on the emitting surface of CNTs. This results from the space charge in the vacuum gap, which is readily generated due to the high emission current density of CNTs. Details are given of both experimental and theoretical studies of this effect. The mechanism also accounts for the distinct nonlinearity in Fowler-Nordheim plots often observed with CNTs. The implication of the technical application of our findings is also included.