To verify the function of orf18,a type Ⅲ polyketide synthase gene isolated from Streptomyces sahachiroi ATCC 33158 was used to construct the knockout mutant strain by in-fame deletion of orf18 and the complementation mutant strain by ectopic expression of orf18 in the knockout strain.The results of HPLC and LC-MS analyses showed that knockout of orf18 significantly reduced the benzoic acid production.Complementation of orf18 recovered the yield of benzoic acid.It is indicated that orf18 is involved in the biosynthesis of benzoic acid in the original host strain.