Streptomyces can produce an overwhelming majority of known antibiotics and several biologically active compounds, but whether Streptomyces can display allelopathic effects on higher plants is largely unknown. In this study, seven actinomyces strains isolated from soils showed inhibitory effect on plant seedlings growth, among which, Streptomyces sp. 6803 had strong capability in inhibiting the seedlings growth of Brassica campestris and Echinochloa crusgalli in both solid and liquid cultures. The dilute solution (x 5) of fermented broth inhibited the seedlings growth of B. campestris and E. crusgalli by 60.7% and 61.3%, respectively. Based on the morphological and physiological-biochemical characteristics and 16S rRNA sequencing, Streptomyces sp. 6803 was identified as Streptomyces arenae, with the 16S rRNA sequence identity being 99.28%. Ultraviolet radiation and diethyl sulfate (DES) were used to produce mutants to enhance the allelopathic potential of this strain. After 80 and 100 seconds of ultraviolet radiation, the dilute solution (x10) of fermented broth of obtained mutants UV8024 and UV100-2 showed 37.5% and 38.1% higher inhibition effect on the root growth of B. campestris seedlings, respectively, compared with the control. The mutant D507 obtained through 1% DES treatment for 50 min showed 29.8% higher inhibition effect on the root growth of B. campestris seedlings. This study showed that Streptomyces sp. 6803 had allelopathic effect on higher plants, and it was possible to enhance the allelopathic potential of the strain via mutation breeding.