Machining characteristics of aluminum nitride by various kinds of the harmonics of Nd:YAG laser, that is the fundamental harmonic (λ₁ = 1064nm), the second harmonic (λ₂ = 532nm), the third harmonic (λ₃ = 355nm) and the forth harmonic (λ₄ = 266nm), were experimentally investigated. Since the material removal is mainly performed by thermal processing in the case of long wavelength such as fundamental wavelength, the removal rate greatly decreases with increasing the distance from the focusing point to the tip of removal. Thus, the limit of removal depth was shallow, since the material removal was sensitive to the energy density of laser beam. On the other hand, when the photon energy of laser beam is larger than the bonding energy of material, the material is mainly removed by the photochemical reaction. Thus, the decrease of removal rate was small for shorter wavelength. Therefore, the removal rate increases with decreasing the wavelength of laser beam for long term irradiation under the same energy condition. In the case of deep hole drilling, the drilled depth increases proportionally with the number of laser shot at the beginning region. Therefore, the removal depth could be controlled by the number of laser shot in this region.