During the screening of inhibitor of DNA topoisomerase I from microbial secondary metabolites, Streptomyces melanosporofaciens 7489 which was capable of producing high level of inhibitor was selected from soil. The active compound (7489-1) was purified from the culture broth by solvent extraction, silica gel column chromatography and HPLC. The inhibitor was identified as dibutyl phthalate by spectroscopic methods of UV, $^{1}H$-NMR, $^{13}C$-NMR, DEPT and EI-MS. 7489-1 showed a strong inhibitory activity against topoisomerase I with 10 ${mu}$M of $IC_{50}$ value.