Seventeen strains of endophytic fungi were isolated from the stems,leaves and fruits of Sorbus tianshanica Rupr.,their antimicrobial activities against two tested microorganisms were investigated by using confront culture method and mycelium growth inhibition tests.The results showed that:(1)Endophytic fungus L18 had strong antimicrobial activities to Fusarium oxysporum f.sp.and Verticillium dahliae Kleb,respectively.Strain L18 was identified as Penicillium polonicum by its morphological features and phylogenetic analysis based on internal transcribed spacer of rRNA sequence comparison.The antimicrobial substance of strain L18 in the fermentation broth was sensitive to high temperature.After 40 ℃ treatment,antimicrobial activity decreased to 59.1% of the control.It was more stable under ultraviolet than light.However,it could be degraded by proteases.The antimicrobial substance could be effectively extracted by ethyl acetate and indicated that it was low polar.Based on these results,we proposed that its antimicrobial substance may be proteins or peptides.The research revealed the basic features of strain L18 and might help to its further study and application,such as the isolation and purification of its antimicrobial substances.