Molten glass containing ruthenium dioxide (RuO2) particles develops electronic conduction, which leads to an unstable operation of the Joule-heated melter for the radioactive waste vitrification. The percolation behavior of molten glass-RuO2 composites was examined by noting their hierarchical formation process. A low percolation threshold < 1 vol% was reasonably reproduced by assuming the packing of "soft" RuO2 rods in the primary aggregate, and by applying Bruggeman's effective medium approximation for the higher aggregates and their clusters.