The effect of nanoclay loading on the crystalline structure and the thermal properties of the polyacrylonitrile-based carbon-polymer-layered silicate nanocomposites was investigated. The modified Montmorillonite with 0–7.5 wt% was distributed by a melt-mixing method in a phenolic matrix and a polyacrylonitrile-based carbon cloth was used as the reinforcement. The composite was prepared by a manual layout method and cured in an autoclave at 160 °C for 4 h. The dispersion of the clay layers in the resin was determined by XRD and TEM studies. The oxyacetylene flame and cone calorimetric tests were used to investigate the ablation performance and the thermal behavior of the nanocomposites. The results showed that the d-spacing of nanocomposites was doubled due to nanoclay loading. The oxyacetylene test showed that nanoclay addition had a better insulation effect. Also, the heat release rate (HRR) of heat was decreased up to 30% by adding the layered silicates.