The capacitance-voltage(C-V) characteristics of organic light emitting diodes(OLEDs) are studied.The effects of hole-injection layer on the negative capacitance of OLEDs are investigated.The results show that the negative capacitance is closely related to the electric field distribution in organic hetero-layer light emitting devices,and the onset frequency of the negative capacitance increases exponentially with the square root of the applied bias.Compared with the device with thin film hole-injection layer,the device with doped hole-injection layer has a lower driving voltage,while its charge transfer characteristic and the onset voltage of negative capacitance are more dependent on frequency.