The purpose of this study aims at standardizing the Mental Scale of Bayley Scales of Infant developemnt, establishing age norms for Chinese infants and toddles and understanding the major characteristics of cognitive function of infants. These knowledge can be for infant caring and clinicat diagosis. The subjects of this study are 1680 infants and they are divided into 14 age groups from one to thirty month old. each subject has been given the mental scale of Bayley Scales of Infant Development. the main findings are as the following: 1. The rate of passing each item in the mental scale of Bayley Scales of Infant Development of the subjects increases along with age according to a linear tendency. 2. The mental scores of the subjects increases with their age according to a linear tendency. 3.Female subjects have higher mental scores than male subjects. 4. In the 3, 4, 21 and 24 month age groups, subjects of city residence have higher mental scores than the subjects of country residence. 5. Ways of caring infants have no effects on the mental scores of the subjects.