This work focused on an improvement of flame retardant properties of acrylonitrile-butadiene-styrene (ABS) by varying amount of silica and organomontmorillonite (OMT). The silica was extracted from agricultural waste such as rice husk by chemical treatment. The obtained silica was characterized by SEM-EDX, XRF, FTIR, laser particle size analysis, and surface area analysis. After that, the rice husk silica (RHS) was surface-modified with 3-aminopropyltrimethoxysilane (AMMO) before blending with ABS and OMT at various ratios by melt blending technique using twin-screw extruder. The effect of silane coupling agent on flammability properties of ABS nanocomposites was studied. The prepared nanocomposites were characterized by TEM to examine the dispersibility of OMT and AMMO-g-RHS in ABS matrix. The flammability properties were evaluated by measuring the limiting oxygen index (LOI) values and UL-94 classes. The TEM results revealed that the ABS/OMT/AMMO-g-RHS nanocomposites were a kind of exfoliated structure. The modified silica particles and OMT were found to disperse and reside in the SAN phase of the matrix. Furthermore, OMT and modified silica could enhance flame retardant properties of ABS polymer matrix because of a synergistic effect between OMT and modified silica during the combustion of ABS nanocomposites.