Electron field emission (FE) from multiwall carbon nanotubes (CNTs) in air for excitation of atmospheric pressure microplasma was investigated in a scanning electron microscope (SEM). The results showed that, the FE properties of the CNTs in air at 10 kPa and lower pressures were the same as those in vacuum. At pressures more than 10 kPa, the FE threshold voltages in air were higher than those in vacuum and increased with increasing air pressures. When the FE threshold voltages became higher than those of the conventional gas breakdown voltage, microplasma was ignited before FE appearance. FE properties of the CNTs were stable in air before appearing the conventional gas discharges.