In this study, the methanolic extract (ME) and the essential oil (EO) of the medicinal plant Artemisia campestris L. were investigated for their antioxidant activity and their antifungal efficacy on the postharvest storage decays; Botrytis cinerea Pers. and Penicillium expansum Link. The total polyphenolic and flavonoid contents were determined. The ME had higher total polyphenolic and flavonoid contents (400.64 µg GAE/mg and 43.13 ± 0.14 μg QE/mg, respectively) than EO (27.47 ± 0.44 µg GAE/mg and 14.04 ± 0.82 µg QE/mg, respectively). The ME presented higher radical scavenging power than the BHT and its IC50 values were 11.71, 40.96 and 23.32 µg/mL for the DPPH, β-carotene bleaching and reducing power respectively. In the antifungal activity, the EO had the stronger effect on both molds, particularly at concentrations > 15 µL, ≥ 800 µL/L and ≥ 15 µL by fumigation, incorporation and disc-diffusion methods respectively, resulting in higher than 80% inhibition of B. cinerea mycelial growth, and from 50 to > 80% inhibition on P. expansum mycelial growth. Methanolic extract showed nearby 50% inhibition on both fungi. The EO MIC was less than 2.5 µL/mL which was shown as MFC for both molds. The bio-autography test has shown separated compounds of the ME having an inhibitor effect on spore germination. These results offer an advantage of suggesting A. campestris could be used as a material for extraction of certain antifungal chemicals for preventing spoilage in food items.