Upon infection, phytopathogenic fungi secrete an array of hydrolytic enzymes that can degrade components of the host epidermis, including waxes, the cuticle, and cell walls. Cellulases, which can hydrolyze crystalline cellulose in the plant cell wall, are among these hydrolytic enzymes. Here, we provide RNAi-based evidence to show that cellulases belonging to glycosyl hydrolase (GH) families 6 and 7 contribute to the penetration of the host epidermis and further invasion by the phytopathogenic fungus Magnaporthe oryzae. The GH6 and GH7 cellulases likely include all members of the cellobiohydrolase family and some endoglucanases in M. oryzae. Quantitative reverse-transcriptase polymerase chain reaction analysis indicated that more than half of the cellulases were highly induced during infection. We constructed knock-down (KD) mutants of these cellulases using the building blocks method we reported previously. The transcript levels of the target genes and cellulase activity were considerably reduced in the KD mutants. The KD mutants resulted in fewer lesions, less penetration, and infection of fewer cells compared with the parent strain. Cytological analyses showed that a high rate of papilla formation blocked invasion of the KD mutants into host cells. These results suggest that the GH6 and GH7 cellulases play roles in the virulence of M. oryzae.