Abstract: Endophytic actinomycete CR20 with antagonistic effect on many plant pathogens was isolated from the root of citrus，it was identified as Streptomyces sp. In order to improve the antibiotic producing ability of CR20，the fermentation medium was optimized using one\|factor\|at\|a\|time and orthogonal test. The results showed that culture medium containing 1.5% glucose， 1% soluble starch， 3% peanut powder， 1% （NH4）2SO4， 0.4% NaCl， 0.2% CaCO3， 0.1% FeSO4·7H2O and 0.1% KH2PO4 was the best， and the inhibition rate of Colletotrichum gloeosporioides from citrus reached to 84.66% and was 1.3 times higher than of that before optimization. Based on the optimized medium，the optimum fermentation time of the strain CR20 is 96 h in 5 L automatic fermentor.

Key words: endophytic actinomycete, fermentation medium, antibiotic