Leather is widely used in various industries including apparel, safety clothing, aircraft and automotive, due to its unique properties such as softness, air permeability, chemical resistance, high flexibility and reasonable mechanical resistance. Since leather products usually contain flammable organic compounds such as tanning, fatliquoring, dyeing and finishing materials, improvement of its flame retardancy is very important. A lot of flame retardants have been synthesized and applied to improve flame retardancy of leather. One of the best materials is nanoclay because it is easily available, environment-friendly and has a low cost. In this research, we propose a process that reduces the burning length of the leather and increases its thermal stability. For this purpose, clay nanoparticles by 1, 3, and 5 mass percent (relative to wet leather mass) were added to wet-blue bovine and goat leather under certain (temperature and time) conditions during the re-tanning process. The results of the TGA test on bovine leather samples showed that by increasing the amount of nanoclay, the thermal stability of samples was increased. SEM images prepared from the grain and cross-sections of bovine and goat leather samples showed that with a high percentage of nanoclay, particle agglomeration is partially visible. The results of the vertical flammability test also showed that the presence of clay nanoparticles reduced the burning length of leather samples. Tensile strength of bovine leather samples containing clay nanoparticles increased compared to the control sample. However, the tear strength of them did not differ significantly.