Dementia among elderly is a serious problem worldwide. This study was conducted to estimate the prevalence and associated risk factors of dementia and mild cognitive impairment (MCI) in nursing homes (NHs) and common communities (CCs) among elderly in China.A cross-sectional survey was conducted in 4 communities across 12 cities in Southern China from May to November of 2014. Qualified psychiatrists and trained nurses carried out relevant diagnosis, assessments, interviews, and information collection. Screening test of mini-mental state examination was conducted among participants firstly, then confirmed diagnosis was carried out among the ones with positive results. Student t test, χ test, univariate, and multivariate logistic regression analysis were conducted to analyze data.A total of 2015 participants aged 65 or older were included in the final analysis; 908 came from NHs while 1107 came from CCs. The crude prevalence rates of dementia and MCI were 22.0% and 15.8%, respectively among all the participants. Dementia prevalence was 42.4% among those living in NHs, which was significantly higher than that of 5.3% in CCs (P < .0001). There were more moderate and severe dementia in NHs compared with CCs (P < .0001). It showed that older age, illiterate compared with high level of education (adjusted odds ratio, AOR = 3.32, 95% CI: 1.53-7.21), heavy drinking (AOR = 1.51 (1.00-2.24), having a medical history of diabetes (AOR = 1.41, 95% CI: 1.02-2.33), and stroke (AOR = 1.21, 95% CI: 1.01-1.23) were associated with dementia in NHs, and middle socioeconomic status might be a protective factor for dementia (AOR = 0.33, 95% CI: 0.21-0.51).The problem of senile dementia in NHs is much more serious than our estimation, and there are not enough trained nursing staffs in NHs. More population-based strategies in NHs, including conducting cognitive screening accompanied with routine physical examination among elderly population, carrying out related primary prevention policies and public health services, and paying attention to some modifiable associated risk factors such as heavy smoking and drinking are needed.