The measurements of forward current-voltage characteristic and forward capacitance-voltage characte-ristic are the most important methods to study the forward electricity characteristic of light-emitting diodes. Forward alternating current (ac) small signal method can be used to measure the capacitance-voltage characteristic of light-emitting diodes

and negative capacitance can be observed. By analyzing the responds of light-emitting diodes junction capacitance by the alternating current small signalmethod based on forward direct current

we found that the current phase of apparent capacitance is behind the phase of voltage when ＜0

leading to apparent capacitance being negative capacitance in measurement. Additionally

the expression of the negative capacitance in LED related with the recombination emitting is firstly obtained. By analyzing the responds of variable capacitance to the alternating current small signal

we also found that the variable capacitance for the specific parameter can make the phase of current be behind the phase of voltage

leading to the negative capacitance in measurement. The result of theory analysis tallies with the experiment. It was found the negative capa-citance is valuable for study the electrical characteristics of light-emitting diodes and valuable for the knowledge to improve the characteristic and parameter relevant the p-n junction internal structure of light-emitting diodes.