The noncoding miRNA-34a (miR-34a) is involved in Alzheimer's disease (AD) pathologenesis and shows potential for application as a biomarker for early diagnosis and intervention. Here, we established miR-34a knockout mice in an APP/PS1 background (APP/PS1-miR-34a KO mice) by crossbreeding miR-34a−/− mice with APP/PS1 mice. We then investigated cognitive impairment and related pathologies. The results showed that the level of miR-34a was increased at about 6 months in APP/PS1 mice, consistent with the increase in amyloid β (Aβ), and cognitive function was significantly improved in mice when miR-34a was knocked out in 9-month-old and 12-month-old mice, indicating that miR-34a is a potential candidate for determining the progression of AD. Furthermore, we assessed the processing of amyloid precursor protein (APP) and the results suggest that cognitive improvement by miR-34a knock out was mainly triggered by depression of γ-secretase activity, without affecting β- and α-secretase activities, indicating that miR-34a plays an important role in AD pathology, mainly by inhibiting the amyloidogenic processing of APP, without altering the non-amyloidogenic processing of APP.