The subacute toxicity of xylooligosaccharide (XO) was evaluated in SD rats. Groups of 60 male and 60 female rats were orally administered with 0, 333, 1000 or 3000 mg/kg of XO for 13 weeks. The changes of body weight, food and water consumption were investigated for 17 weeks, while heamatological values and histopathological findings were investigated at the end of the 13 weeks and 17 weeks including 4 weeks of recovery periods. No death and toxic effects were observed during the test periods. There were statistically significant changes in several parameters, but these change had no direct relationship to dosage. Clinical changes were general occurrence and no specific toxicity was related to XO. Gross necropsy and histopathology revealed that no target organs were found in the treated mouse with XO. According to the results, no-observed effect level of XO is estimated to be above 3000 mg/kg.