We studied age related correlations of mismatch negativity (MMN) auditory event-related potentials (ERPs) using odd ball paradigm in 121 normal subjects (from 6 months to 43 years) and 33 mentally retarded subjects (from 2 to 18 years). The subject was instructed to read a book in order to ignore the stimulus during the experiment, or no specific task was imposed on young child. The MMN wave form was clearly obtained in normal 6 month old subjects. As the age advanced from 6-months, the latencies of MMN progressively shortened till about 7 years, when the latency reached adult level. One-third of mentally retarded patients had prolonged MMN than that normal age-matched subjects. Reliable recording of P300 requires subject's cooperation and attentiveness, which are difficult to achieve in young children or mentally retarded patients. MMN may be utilized in children to evaluate the development of the cognitive function.