The use of a differential capacitance technique for characterizing the relaxation-induced defect states in Schottky diodes has been studied. Based on a proposed equivalent circuit including the effect of potential drop across the carrier-depletion layer, a simple equation of capacitance at different voltages and frequencies is derived and compared with experimental data obtained from relaxed In0.2Ga0.8As/GaAs samples. It is shown that the carrier-depletion layer will introduce capacitance dispersion over frequency like traps; from it the device's geometric parameters, the resistance of the carrier-depletion layer and the ionization energy of the deep level that gives rise to this resistance can be obtained. The relation between the low-frequency capacitance and reverse voltage can be well explained by the depletion of the free carriers between the Schottky depletion and the carrier-depletion layer. The relaxation-induced traps are believed to be at 0.535 and 0.36 eV, respectively, in the GaAs and In0.2Ga0.8As regions.