This study presents aquick and accurate method for analysing a filter circuit in the time and frequency domains. A computer program called FAP was written and is able to provide the complete set of frequency and time responses as well as the location of poles and zeros. The program is based upon a system theoretic approach. This approach is used because the relationship between excitation and response is an algebraic one. Fap has many important uses and these are describe clearly together with relevant examples. To provide a better understanding of FAP, the program is divided into various sections each complete with flowchart and explanation of its purpose. In order to test the capability of the program, a third orde Butterworth filter network was used. The complete set of responses for the ideal filter was obtained and their main features were described. Then the performance of the filter with parasitic capacitance was studied and the results compared with those of the ideal filter. The main feature in the frequency response of the filter with the stray capacitances is is the presence of a notch. The changes in the performance of this filter were discussed in relation to the ideal filter. FAP is econimical from the point of view of storage and execution time. FAP can actually handle linear circuit. The only input required is the transfer function.