The liquid culture of Laetiporus sulphureus var. sulphureus was lethal against fruit fly. It was found that extracellular metabolites were primary causation of the lethal effect against fruit fly, which was influenced by pH value. Isolation and analysis with ion-exchange resin column chromatography and HPLC demonstrated that oxalic acid was present in supernatant of Laetiporus sulphureus var. sulphureus, and it was one of the contributing factors to lethal effect against fruit fly and decrease of pH value of culture system. When cultured in airlift reactor, concentration of oxalic acid, quantity of mycelia and pH value was correlated with each other. Further analysis on elution revealed that a kind of oligidic pigment of amaranth in alkaline condition were also lethal against fruit fly.