Assuming aftershocks concentrating in the center of fault planes and obeying three-dimensional（3D） normal distribution, the authors reconstructed a 3D structure of the active part of a fault network by using a fuzzy clustering and principal component analysis. When the seismic events are given, the optimal spatial fault segments can be obtained by this method. The authors also applied the method to the determination of the fault plane from the aftershock sequence of Landers earthquake in southern California. The reconstructed plane segments fully agree with the faults already known on geological maps or with the blind faults appearing to be quite clear.