The deep trap levels of AlxGa1-xN films with x in the range from 0 to 0.15 grown on c-plane sapphire substrates using rf-plasma-assisted molecular-beam epitaxy have been investigated by deep level transient spectroscopy measurements. Two distinct defect levels (denoted as Ei and Di) were observed. The origins of the Ei and the Di are associated with point defects such as the N vacancies and extended defects, such as the threading dislocations, respectively. According to Al content (x), the activation energy and capture cross section for the Di defect ranged from 0.19 to 0.41 eV and 1.1–6.6×10-15 cm2, respectively. The trap energy levels of Di defects in AlxGa1-xN were calculated and the values were nonlinear with Al content. The bowing parameter of AlxGa1-xN films was determined to be b=1.22.