Objective. To explore the changes of three- tone paradigm P300 and observe the improvement of the diagnostic sensitivity of P300 in aging dementia. Methods. Thirty- four patients of aging dementia underwent traditional P300 （2t- P300） and three- tone paradigm auditory P300 （3t- P300）. The results of these two methods were compared. Results. Abnormal P300 was found in 18 of 34 dementia cases （52.9％ ） by 2t- P300

and in 26 of 30 patients who finished the 3t- P300 tests （86.7％ ）. In 2t- P300

9 of 24 mild cases

7 of 8 moderate cases and 2 severe cases showed abnormal results. In 3t- P300

20 of 24 mild cases and all 6 moderate cases showed abnormal results. Abnormal 3t- P300 was found in 13 of 16 cases which showed normal 2t- P300. Besides prolonged and bad- shaped P300

no certain response of P300 to rare non- target stimulus （r- nT） was found in 10 mild cases by 3t- P300. Conclusion. Three- tone paradigm auditory P300 had higher sensitivity in early diagnosis of aging dementia than traditional auditory P300. No P300 response to r- nT in 3t- P300 is an important index in early diagnosis of mild dementia.