Activated carbon (AC) filled epoxy nanocomposites obtained by mixing the desired amount of nano AC viz., bamboo stem, oil palm empty fruit bunch, and coconut shell from agricultural biomass with the epoxy resin. Flexural properties of activated carbons filled epoxy nanocomposites with 1%, and 5% filler loading were measured. In terms of flexural strength and modulus, a significant increment was observed with addition of 1% vol and 5% vol nano-activated carbon as compared to neat epoxy. The effect of activated carbon treated by two chemical agents (potassium hydroxide and phosphoric acid) on the flexural properties of epoxy nanocomposites were also investigated. Flexural strength of activated carbon-bamboo stem, activated carbon-oil palm, and activated carbon-coconut shell reinforced epoxy nanocomposites showed almost same value in case of 5% potassium hydroxide activated carbón. Flexural strength of potassium hydroxide activated carbon-based epoxy nanocomposites was higher than phosphoric acid activated carbon. The flexural toughness of both the potassium hydroxide and phosphoric acid activated carbon reinforced composites range between 0.79 – 0.92 J. It attributed that developed activated carbon filled epoxy nanocomposites can be used in different applications.