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Percolation clusters with varying occupation probability were constructed Viscous fingering (VF) in the percolation cluster, based on the assumption that throat radii are Rayleigh distributed, is investigated by means of a successive overrelaxation technique. The fractal dimension and the sweep efficiency of VF in the percolation cluster when surface tension is considered are larger than when surface tension is neglected. The fractal dimension of VF will increase as the percolation probability increases or the viscous ratio decreases. VF's fractal dimension of porous media in the limit viscous ratio --> infinity is found to be identical with the DLA, The topology and the geometry of the porous medium have a strong effect on the displacement processes and the structure of the VF. (C) 1999 Elsevier Science B.V.