Nematode-trapping fungi live mainly as saprobes in soil environments. When encountering nematodes, these fungi become ‘carnivorous’ and develop specialized trapping devices to attack their hosts for extracting nutrients, especially nitrogen source. Thus, nematode-trapping fungi are model organisms for understanding the molecular mechanism of the switch between saprobic and parasitic phases of pathogen life cycles. Arthrobotrys oligospora, one of the best-studied nematode-trapping fungi, mainly lives as a saprobe. In the presence of nematodes, A. oligospora enters the parasitic stage by forming adhesive reticulate traps to capture nematodes. In filamentous fungi, autophagy has been shown to be involved in morphogenesis and morphology. In this study, we demonstrate that autophagy is induced by nematodes during the early stage of trap formation in A. oligospora. Disruption of atg8 gene not only abolishes the nematode-induced autophagy, but also suppresses trap formation and reduces pathogenicity for nematodes. During the early stage of trap formation, the expression of genes involved in amino acid biosynthesis is upregulated and the transcriptional activity of GCN4 is induced in A. oligospora, suggesting that nematodes induce autophagy probably by triggering intracellular amino acid starvation. Autophagy is thus crucial for trap formation in A. oligospora during infection of nematodes.