Emerging data suggests that mitochondrial dysfunction is prominently involved in Alzheimer disease (AD) progression. Sirtuin-3 (Sirt3) is a member of the sirtuin family of nicotinamide adenine dinucleotide dependent deacetylases that regulates a variety of mitochondrial functions and suppresses mitochondria-related physiology. Here, we determined sirt3 expression in a mouse model of AD. Spatial learning and memory were tested by Morris water maze in APP/PS1 double transgenic mice. The expression of sirt3 was assayed by real-time quantitative PCR and western blotting. Age-and gender-matched wild-type (WT) littermates were used as controls. Cortical sirt3 localization was assessed using immunohistochemistry. The expression of sirt3 mRNA was significantly lower in the cortex of APP/PS1 double transgenic mice than in WT littermates (0.83 ± 0.24 vs. 1.10 ± 0.21, P < 0.05). A comparable reduction was found in sirt3 protein levels using western blotting. The ratio of mean optical density (MOD) of total sirt3/β-actin in the cortex was 0.77 ± 0.11 in APP/PS1 double transgenic mice and 1.34 ± 0.17 in the WT littermates (P < 0.01). Immunohistochemistry showed the same change as western blotting. The ratio of MOD of integral optical density/total area in APP/PS1 and WT littermates was 0.58 ± 0.02 and 0.71 ± 0.05 (P < 0.01). These data show that sirt3 was depleted in APP/PS1 double transgenic mice. The results suggest that mitochondrial sirt3 might participate in the development of AD via mitochondrial dysfunction.