A.c. electrical properties of the heterostructured polymer light-emitting diode consisting of poly［2-methoxy-5-(2′-ethyl-hexyloxy)- 1, 4-phenylenevinylene］(MEH-PPV) as the light-emitting layer, and poly［(9,9 - bis(3′-(N,N-dimethylamino) propyl)-2,7- fluorene)-alt-2,7-(9,9-dioctylfluorene)］ (PF-NR2) as the electron-injecting layer, are studied by using impedance spectroscopy. Negative capacitance in impedance spectroscopic data has been observed. According to the Cole-Cole plot of the device ITO/PEDOT/MEH-PPV/PF-NR2/Al, the a.c. response of MEH-PPV/PF-NR2 interface can be explained in terms of the equivalent circuit model of a parallel combination of an inductance L and a resistance RL. The experiments showed that PF-NR2 could act as an electron injecting and hole blocking layer. We propose an energy band structure schematic diagram to explain the enhancement of the EL efficiency in the ITO/PEDOT/MEH-PPV/PF-NR2/Al structure.