The work functions of multi- and single-walled carbon nanotubes are found to be 4.95eV and 5.10eV respectively. The measurements were carried out using the photoelectron emission (PEE) method, which allows easy and precise measurements to be made in air. We have found that the work function of the nanotubes is 0.1–0.2eV larger than that of highly oriented pyrolytic graphite (HOPG) of which the valence state is σ–Φ orthogonal. This result is ascribable to reflection of the σ–Φ⊏ mixed valence state in the case of carbon nanotubes. The experimental data were well reproduced in ab-initio calculations on planar and cylindrical conjugated states.