Objective To analyze the characterization of cognitive function in Parkinson's disease with dementia and Alzheimer's disease. Methods Cognitive function was examined in Parkinson's disease with dementia (PDD) patients ( n = 30) , Alzheimer's disease (AD) patients ( n = 30) and healthy elderly control subjects ( n = 60) . Neuropsychological evaluation contained semantic fluency test, phonemic fluency test, action fluency test, objective and action naming tests. Results In PDD group , the score of semantic fluency test is 9. 33 ±2. 78, 6. 17 ± 1.67 of phonemic fluency test and 7.03 ±2. 34 of action fluency test,it is 6.90 ±2.47, 7.87±2.01,8.30±3. 17 of AD group. The score of objective and action naming tests is 36.33 ±3.39, 17.63 ±2. 17 in PDD group,while AD patients is 33.23 ±3.56 and 22.33 ±2.37. The verbal fluency tests and naming tests were impaired in PDD and AD patients compared with the healthy elderly control group (P ＜ 0. 01 ), phonemic fluency, action fluency and action naming were more impaired in PDD patients compared with the AD group , while semantic fluency and objective naming were more impaired in AD patients (P ＜ 0. 01 ). Conclusions Executive function deficit and naming impairment are found in PDD and AD patients, it shows that PDD is characterized by the addition of cortical dysfunction upon a predominant and progressive fronto-subcortical impairment. There is subcortical dysfunction in AD patients.