The mixture of peppermint oil (PO) with medium-chain triacylglycerol was emulsified in water and stabilized with a food-grade biopolymer, modified starch, to form PO nanoemulsions. The effects of emulsifying conditions including homogenization pressure, the number of processing cycles, and oil loading on the mean diameters and viscosities of nanoemulsions were characterized by dynamic light scattering, optical microscopy, and rheological measurements. The formulated PO nanoemulsions with mean diameters normally <200 nm showed high stability over at least 30 days of storage time. Their antimicrobial properties related to those of PO have also been evaluated by two assays, the minimum inhibitory concentration (MIC) and time-kill dynamic processes, against two Gram-positive bacterial strains of Listeria monocytogenes Scott A and Staphylococcus aureus ATCC 25923. Compared with bulk PO, the PO nanoemulsions showed prolonged antibacterial activities. The results suggest that the nanoemulsion technology can provide novel applications of essential oils in extending the shelf life of aqueous food products.