This preliminary study is aimed to investigate whether it is feasible to conduct a study to use fish oil compared to the placebo(olive oil) in people with cognitive impairment. We will also explore whether fish oil has better efficacy in some clinical aspects in people with cognitive impairment during 24 weeks intervention. The major clinical outcome will be:

1. general clinical impression
2. cognitive function

Despite some positive findings from observational and animal studies, the effects of n-3 PUFA administration on cognitive impairment in humans have received little evaluation to date. Although some clinical trials of fish oil have been reported, the results are inconsistent. Given these inconsistent findings, we carried out a preliminary study to investigate the effect of fish oil monotherapy on cognitive function and general clinical condition in patients with cognitive impairment.

Inclusion Criteria:

• fulfilled the diagnosis of AD according to the American Psychiatric Association, DSM-IV criteria, with mild or moderate severity( defined by an Mini Mental Status Examination (MMSE) score between 10 and 26, and a Clinical Dementia Rating (CDR) score of 1 or 2.)
• or amnesic MCI( defined as (1). Subjective memory impairment by the patient and /or an informant, (2) objective memory impairment falling at least 1.5 standard deviations or more below age- and education-specific norms on the Logical Memory delayed-recall score from the Wechsler Memory Scale III (3) relatively normal performance in other cognitive domains, (4) no impairment in activities of daily living, and (5) failure to meet DSM-IV criteria for dementia.)

Exclusion Criteria:

• inadequate motor or sensory capacity to comply with testing
• any ischemic lesion on brain CT reported by the radiologist or a modified Hachinski Ischemic Scale score >4
• a 17-item Hamilton Depression Scale (HDRS)score > 13
• abnormal levels of folic acid, vitamin B12, or thyroid function
• severe comorbidity, including another neurodegenerative diseases, another chronic debilitating neurological illness (e.g. cerebral palsy), brain trauma, tumors, severe pulmonary, renal, liver disease, cardiac disease, or autoimmune disease, or conditions expected to cause death within one year.
• Participants with a diagnosis of alcoholism, schizophrenia, and bipolar disorder were excluded.
• Participants receiving cholinesterase agents during the screen or taking NSAID on a long-term basis