Polarization-induced charges in modulation-doped Al0.22Gao.78N/GaN heterostructures were investigated by the capacitance-voltage (C-V) method. The C-V profile of the Pt/Al0.22Ga0.78N/GaN Schottky diodes with various Al0.22Ga0. 78N thicknesses shows significant differences due to change of the polarization field in the heterostructures. A numerical simulation based on the experimental results indicates that the sheet density of the polarization-induced charges at the heterointerface is 6.78.× 1012 cm- 2 in the samples with the Alo.22 Ga0.7sN thick nesses of 30nm or 45nm. The charge density reduces to 1.30 × 1012 cm-2 in the sample with the Al0.22Ga0.7sN thickness of 75 nm. It is thought that the reduction of the polarization-induced charges at the heterointerface is due to the partial relaxation of the Al0.22Ga0.7sN layer on GaN. This work provides a technique for quantitative characterization of the polarization-induced charges in AIx Ga1-xN/GaN heterostructures.