Films of Japanese lacquer with soybean, linseed or Japanese tung oil additions were investigated for their hardening process and properties of their hardened films. (1) When a drying oil was added to Japanese lacquer, the absorption band at 174Ocm_-1 attributed to > C = O group (gammaC = O) shifted to 1715 cm_-1, and the absorbence at 1740cm_-1 sharply increased upon the hardening of the film. This phenomenon which had been already found in Japanese lacquer film added with methyl oleate, was proved to be due to the formation and disappearance ofhydrogen bonding betweenthe > C = O group of the oil and -OH group of urushiol, main constituent of Japanese lacquer. (2) The addition of drying oils retarded the hardening of Japanese lacquer. The retardation tendency was in the following order: soybean oil>tung oil linseed>oil. Adding oils also lowered the elastic modulus of the hardened films. In view points of drying rate, luster and elastic modulus of the hardened films, linseed oil is the most suitable one to be added to Japanese lacquer.