1168 healthy children of 3-13 years old were selected as subjects from Beijing, Shanghai, Guangzhou and Haerbin. Total subjects were challenged by 25 gram lactose test. The breath hydrogen was determined, the symptoms of lactose intolerance were recorded. The results showed that the incidences of lactase deficiency of children of 3-5, 7-8 and 11-13 years old were 38.5%, 87.6% and 87.8% respectively. The incidences of lactose intolerance were 12.3%, 33.2% and 30.5% respectively. Lactase activity decreasing of Chinese children occurred at the age of 7-8 years old, and the symptom of lactose intolerance depended on lactose dose. But, there still are 39%-41.7% children with lactose maldigestion and 14%-16% children with intolerance symptom, after taking 50 gram milk powder (12 g lactose). The relationship between lactase activity and milk intake was not found.