Composite films of carbon nanotubes (CNTs) and MgO nanoparticles are fabricated on metal substrates as field emitters by a simple mechanical rubbing method. The field emission properties of the CNT films could be dramatically improved by mixing an optimum proportion (∼7 wt%) of MgO nanoparticles. It lowers the turn-on field to 1.3 V µm−1 and the threshold field to 2 V µm−1, and also increases the field emission stability and uniformity. Such CNTs–MgO films are applied as cathode in ionization vacuum gauges. The gauges, with the advantages of small size and low power consumption, exhibit good measurement linearity in vacuum pressure range from 8 × 10−6 to 2 × 10−2 Pa, and a sensitivity of 0.2 Pa−1.