Botrytis cinerea  Pers.; Fr., induces grey mold of onion worldwide. Fungicide applications are often used to control this pathogen. Natural plant extract do not show hazardous effects and likely to be used instead of the synthetic fungicides. In this study, antifungal activity of 5, 10, 20 and 40 μL of Mentha longifolia, Origanum marjoram, Artemisia sp and Trichoderma filtrate, respectively were applied in vitro to study their effect on the mycelial growth of B. cinerea using a poisoned food method. All treatments significantly presented their effect on the mycelia growth of the pathogen while the maximum inhibitory effect was shown when 40 μL of the extract was used. The inhibitory effect was affected by the amounts of the extract and the incubation time of the samples. Samples treatment of 40 μL concentration showed a maximum inhibition in the mycelia growth 5 days after incubation. This study shows that Artemisia sp has a potential to be used in the controlling management of B. cinerea. According to the available literature, this study could be the first to record the potential of Mentha longifolia, Origanum marjoram, Artemisia sp extracts and Trichoderma filtrate on the control of Botrytis cinerea, the causal agent of gray mold of onion in El-Beida, Libya.