Abstract: Trichoderma harzianum is an excellent biocontrol agent with a well-known ability to produce a wide variety of antibiotic polyketides,but there is no relevant report on genes encoding polyketide synthase (PKS) from Trichoderma harzianum.In this study, a full-length coding sequence of PKS (pkst-1) gene of T.harzianum 88 was obtained from clones performed by In-Fusion technique,a plasmid expression vector coding for RNAi targeting mRNA of pkst-1 was constructed successfully,and the expression of pkst-1 gene was inhibited successfully by RNAi.The inactivation of pkst-1 gene made the fermentation of the transformant have a similar effect of inhibiting bacteria as gene-knockout,which led to a reduced inhibitory ability in Trichoderma harzianum,indicating that pkst-1 was the key enzyme of polyketide synthesis pathway.Additionally,the successful application of RNAi in this study provide new perspectives on the function identification of secondary metabolic genes from T.harzianum or other filamentous fungus.