The objective of this study was to investigate contribution of nanoclays to the flame retardancy of polyamide-6 and its 15 wt% short glass fiber-reinforced composite with and without an organophosphorus flame-retardant aluminum diethylphosphinate. Nanocomposites were compounded by melt mixing method via twin-screw extruder, and the specimens for testing and analyses were shaped by injection and compression molding. Transmission electron microscopy and x-ray diffraction analyses indicated that nanoclay layers had intercalated/exfoliated morphology in the polyamide-6 matrix. UL-94 vertical burning, limiting oxygen index, and mass loss cone calorimetry revealed that use of only 5 wt% nanoclays could improve many flammability parameters, such as peak heat release rate. Contribution of silicate layers was much more significant when 5 wt% nanoclays were used together with 15 wt% aluminum diethylphosphinate. Various analyses clarified that the basic flame retardancy mechanism of nanoclays was the formation of insulative barrier via tortuous pathway of silicate layers preventing the underlying polymer from heat and mass transfer during fire. It was also found that all the mechanical properties lost due to the use of aluminum diethylphosphinate could be compensated when its 5 wt% was replaced with nanoclays.