Alzheimer's Disease (AD) is the most common and destructive neurodegenerative disorder threatening old people. As a non-invasive way to assess brain function, the fMRI technique can be used to detect how the brain activity pattern of AD patients differ from that of age-matched elder controls (EC). We compared the brain activity pattern of AD and EC under three conditions: unimodal auditory stimuli, unimodal visual stimuli and bimodal audiovisual stimuli. It was found that patients with AD exhibit a more extensive but relatively weaker response than EC, and in every type of stimuli, the activated brain areas are dissimilar with EC. The limbic lobe of AD patients is universally silent while the hippocampal areas in EC are active. Moreover, compared with EC, AD patients show fewer activated regions for audiovisual integration, indicating impaired multisensory information processing and cognitive integration. The findings imply that fMRI can help with the diagnosis of AD even in the early stage.