Abstract: A fault location method for nonlinear circuit by taking the cores of  Volterra series in frequency domain as the fault characters is proposed.Via calculating the frequency cores of Volterra series in frequency domain for the response of usual fault states and feeding them into BPNN as the fault features，the fault dictionary is built.The fault location for nonlinear analog circuit is completed by BPNN classifying the Volterra frequency cores tested from the nonlinear network.The paper presents the unified recursive computing formulae for frequency cores，an improved BPNN algorithm and its program.At last，an example of this method is given.

Key words: fault location, nonlinear circuit, frequency core, neural network