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Gray mold disease caused by <i>Botrytis cinerea</i> is a devastating disease that leads to serious financial loss. In this study, the entomopathogenic fungus <i>Metarhizium anisopliae</i> that acts against the gray mold pathogen <i>B. cinerea</i> was evaluated. <i>M. anisopliae</i> produced a significant inhibition zone in front of the <i>B. cinerea</i> colony in the dual culture test. In addition, volatile organic compounds generated by <i>M. anisopliae</i> were shown to have an inhibitory effect on <i>B. cinerea</i> mycelia growth and reduced 41% of gray mold severity of postharvest tomatoes. The 10% concentration of the culture filtrate of <i>M. anisopliae</i> inhibited 88.62% of colony radial growth as well as 63.85% of sclerotia germination and all conidia germination of <i>B. cinerea</i>. Furthermore, the culture filtrate of <i>M. anisopliae</i> retained its inhibitory effect against the radial growth of <i>B. cinerea</i> even after heating for 15 min at 100 &#176;C. Feasible mechanisms of <i>M. anisopliae</i> involved in the control of <i>B. cinerea</i> were explored, and it was demonstrated that the plasma membrane of <i>B. cinerea</i> conidia was damaged by the product of metabolism of <i>M. anisopliae</i>. In addition, after treating with culture filtrate of <i>M. anisopliae</i>, the <i>B. cinerea</i> phenotype was shown to be abnormal, and cell organelles of <i>B. cinerea</i> mycelia were damaged significantly. A significant control efficacy of <i>M. anisopliae</i> against tomato gray mold was detected on both the detached leaf assay (84.24%) as well as the whole plant (72.38%). In addition, a 78% reduction in tomato fruit mold was detected at a 10% treated concentration of <i>M. anisopliae</i>. These findings suggest that <i>M. anisopliae</i> possesses potential as a biocontrol agent against tomato gray mold in the greenhouse and during the postharvest stage.