Abstract: Piriformospora indica (P. indica), a root-colonizing endophytic fungus, can improve the resistance of plants against biotic and abiotic stresses. In order to clarify the influences of P. indica on the disease resistance of Nicotiana tabacum, the resistance experiments against tobacco brown spot, anthracnose, damping off and sore shin were conducted wherein P. indica-colonized N. tabacum was inoculated with Alternaria longipes (A. longipes), Colletotrichum gloeosporioides (C. gloeosporioides), Pythium ultimum (P. ultimum) and Rhizoctonia solani (R. solani), respectively; and the contents of malondialdehyde (MDA) and proline, antioxidant enzyme activity and the expression levels of pathogenesis-related protein genes in tobacco leaves inoculated with A. longipes were analyzed. The results showed that comparing with the control, the size of disease spots and the MDA content in the leaves of P. indica-colonized N. tabacum inoculated with A. longipes reduced significantly, while the proline content and the expression levels of pathogenesis-related protein genes PR-1a, PR2, PR3 and PR5 increased significantly; the size of disease spots in the leaves of P. indica-colonized N. tabacum inoculated with C. gloeosporioides reduced; and damage symptoms of the leaves of P. indica-colonized N. tabacum inoculated with P. ultimum and R. solani significantly decreased. P. indica can improve the disease resistance of N. tabacum through maintaining the integrity of cell biomembrane system, keeping intracellular osmotic pressure and the peroxidation of membrane lipid at a lower level and regulating the expression of pathogenesis-related protein genes.