Abstract: The amplitude anomalous absorption of an O-mode low-power diagnostic radio wave passing through the perturbed region in ionospheric modification is investigated. Kinetic theory and Vlasov equation describing electron dynamics are employed to derive the coefficient of anomalous absorption caused by artificial field-aligned irregularities, considering effects of a magnetic field on the dispersion relationship of EM waves and on motions of electrons in calculations of the displacement current density. With analytical expressions, variations of the anomalous absorption coefficient with the geomagnetic dip are calculated. It is shown that the coefficient increases with the geomagnetic dip until 70o, and then drops off slowly. With consideration of the geomagnetic field when calculating the HF conductivity, the coefficient is decreased and more obvious at high geomagnetic dip.