Effects of heat-transferring property of silver nanoparticles were evaluated on two solid woods of poplar and fir and in combination with Borax. Nanosilver and Borax were applied in pressure vessel using Rueping method. The size range of silver nanoparticles was 20-80 nm. Specimens of 150× 130× 9 mm were prepared and five fire-retarding properties were measured using a newly designed slide fire test apparatus. The obtained results indicated that most fire-retarding properties of Borax were improved in combination with nanosilver suspension. Some improvements, although not significant in most cases, were also observed in those specimens that were only impregnated with nanosilver. Significant difference was observed in the two species. Fire-retarding properties were best improved by nanosilver+ Borax impregnation in poplar; however, Borax-impregnated specimens seemed to have the optimum properties in fir.