Anaerobic digestion (AD) is a sustainable waste-to-energy method for converting organic wastes to methane. In this work, a pretreatment of food waste (FW) with free nitrous acid (FNA) was proposed, and various pretreatment times (4 hrs, 72 hrs, and 28 days) and FNA concentrations (1.07, 2.13, and 5 mg FNA-N/L) were tested to evaluate its effectiveness in stabilizing FW and sewage sludge (SS). Soluble chemical oxygen demand (sCOD), soluble protein (sP), and soluble polysaccharide (sPS) in the groups with 50%:50% (w/w VS) pretreated at 5 mg FNA-N/L for 72 hrs were significantly increased by 631, 530, and 780 %, respectively, compared with the control group without FNA addition. Additionally, less sulfide was detected in the groups with FNA addition, indicating an effective reduction in sulfide-odor during the storage/pretreatment of FW and SS. The potential of biogas production of FNA-pretreated FW was assessed using biochemical methane potential (BMP) test, and the results showed that the groups with FNA pretreatment produced up to 91.97% more methane than the group without FNA. The results from this work indicated an improved digestibility of FW and/or SS for AD using FNA pretreatment, and suggested the optimal pretreatment condition at 2 mg FNA-N/L for 24 hrs for FW and FW/SS.