The stabilizing effect of fructooligosanharide (FO) on hen＇s egg yolk immunoglobulin (yIgG) by heat and acid was investigated. The heat stability of yIgG at 70∼80$^{circ}C$ was enhanced in a concentration-dependent manner by adding 0∼50% (w/v) FO to a yIgG solution. Acid-induced inactivation of yIgG was also suppressed in a concentration-dependent relationship by addition of FO. Addition of 50% FO almost completely stabilized yIgG at pH 3. The remarkable stablizing effect of FO on yIgG may enhance the use of yIgG as functional food ingredients.