In this study, we investigated the changes in the color of fire-retardant-impregnated and surface-coated sugi sapwood during 60 months of outdoor exposure. The fire-retardant chemicals used primarily consisted of guanidine phosphate and were modified with phenolic additives to prevent the chemicals from leaching. We adopted the following four coating systems：① penetrating；semi-transparent（brown）, ② thin-film-forming；semi-transparent（ brown）, ③ film-forming；opaque（white）, and ④ film-forming；transparent. Specimens finished with the coating system ③ showed few changes in color difference during 60 months of outdoor exposure, with no significant difference between fire-retardant-treated and untreated samples. The changes in the color difference of fire-retardant-treated samples were smaller than those of the untreated samples when uncoated or coated with one of the coating systems ①, ②, or ④. These results indicate that the fire-retardant treatment used in this study can inhibit the color changes of uncoated and coated wood.

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