Amyloid precursor protein (APP) has eight potential phosphorylation sites in its cytoplasmic domain. Recently, it has demonstrated that the constitutive phosphorylation of APP at T668 (APP695 isoform numbering) was observed specifically in the brain. Neuron-specific phosphorylation of APP at T668 is thought to be important for neuronal functions of APP, although its exact physiological significance remains to be clarified. In this study, we show that the phosphorylation of the APP intracellular domain (AICD) at T668 is essential for its binding to Fe65 and its nuclear translocation and affects the resultant neurotoxicity, possibly mediated through the induction of glycogen synthase kinase 3beta and tau phosphorylation by enhancing the formation of a ternary complex with Fe65 and CP2 transcription factor. Taken together, these results suggest that the phosphorylation of AICD at T668 contributes to the neuronal degeneration in Alzheimer's disease (AD) by regulating its translocation into the nucleus and then affects neurodegeneration; therefore, the specific inhibitor of T668 phosphorylation might be the target of AD therapy.