This study was aimed to determine the effects of a plasma jet of mixed argon and oxygen (Ar‐O2) on Aspergillus flavus growth in potato dextrose broth (PDA). Settings for power, discharge duration, and gas flow rate of the plasma jet system (PJS) were optimized over ranges of 10–20 W, 1–10 min, and 3–9 L/min, respectively. The optimal conditions required to inhibit 100% of A. flavus growth at 30°C for 48 hr were 20 W, 10 min, and 9 L/min, respectively. The PJS inhibited 100% of the A. flavus growth in 20 replicates of infected peeled peanuts. Aspergillus flavus cell walls were destroyed via lipid and cell membrane oxidization, resulting in lost osmotic capability and damaged DNA and enzymes. However, the PJS is limited by a short exposure distance. More power, longer exposure, and increased gas flow might be needed to increase the distance while maintaining 100% microbial growth inhibition. Application to real foods and agricultural products, and upscaling the PJS for industrial use will be assessed in further studies. Practical applications Our proposed plasma system could be applied to postharvested agricultural products such as dried peanuts and chili. However, the technology will require validation before it can be applied in an industrial scale.