Carbon aerogels has particular application in many fields for its nano-sized porous structure. To improve the pore size controllability in the fabrication of carbon aerogels, strategy through increase the formaldehyde concentration to enhance the structural stability of organic resorcinol--formaldehyde (RF) aerogel was proposed in this paper. The RF starting resolution was catalyzed by sodium carbonate in distilled water. The ratio of resorcinol to formaldehyde (R/F ratio) was adjusted from 1/2, 1/1.5, 1/2, 1/2.5 to 1/3. The RF aerogols was derived from wet gel through supercritical drying, and they were carbonized to prepare carbon aerogels. Nano-structural property of the RF aerogels was analyzed by the shrinkage and density. The pore size was discussed by the measurement results of nitrogen gas adsorption. Experimental results show that with an increasing in the concentration of formaldehyde in the starting solution, the structural stability of the RF aerogels increases. It was proved that both the shrinkages and densities of the dried RF aerogels are decreased simultaneously, and that the pore size distribution curves of the RF aerogels are increased steeply through a high concentration of formaldehyde. Therefore, it is tested that the pore size controllability of carbon aerogels is increased with an enhancement in the stability of their RF aerogel precursor.