Abstract: L. fungicola and M. perniciosa are two common fungal diseases of the cultivated mushrooms. In this study, S. hortensis oil was used to evaluate its effectiveness against L. fungicola and M. perniciosa. Pure cultures of the L. fungicola, M. perniciosa, A. bisporus and P. eryngii were initially grown on PDA. Antifungal activity was tested by mycelium radial growth inhibition method on PDA in 90 mm Petri dishes supplemented with 100 to 2000 μl L-1 of S. hortensis oil. The cultures were incubated at 25oC in the dark. This experiment was conducted as completely randomized design with three replications. Probit analysis used for IC50 value calculation. The IC50 of investigated oil against L. fungicola and M. perniciosa were 476.8 and 109 μl L-1, respectively. These amounts for A. bisporus and P. eryngii were 91.1 and 386.2 μl L-1, respectively. The results revealed that the tested essential oil had antifungal activity. Summer savory oil with SI = 0.28 can be a selective oil with potential use in king oyster mushroom production in order to controlling M. perniciosa.