•Tau oligomer plays a critical role in tau pathology.•Tau oligomers become an important therapeutic target for tauopathies.•For now, there is no evaluation method for tau oligomerization in vivo.•By introducing BiFC techniques to tau, tau-BiFC mouse model provides spatial and temporal resolution of tau oligomerization in the brain Accumulation of abnormal tau aggregates in the brain is a pathological hallmark of multiple neurodegenerative disorders including Alzheimer’s disease. Increasing evidence suggests that soluble tau aggregates play a key role in tau pathology as neurotoxic species causing neuronal cell death and act as prion-like seeds mediating tau propagation. Despite the pathological relevance, there is a paucity of methods to monitor tau oligomerization in the brain. As a tool to monitor tau self-assembly in the brain, we generated a novel tau transgenic mouse, named TauP301L-BiFC. By introducing bimolecular fluorescence complementation technique to human tau containing a P301L mutation, we were able to monitor and quantify tau self-assembly, represented by BiFC fluorescence in the brains of transgenic TauP301L-BiFC mice. TauP301L-BiFC mice showed soluble tau oligomerization from 3 months, showing significantly enriched BiFC fluorescence in the brain. Then, massive tau fragmentation occured at 6 months showing dramatically decreased TauP301L-BiFC fluorescence. The fragmented tau species served as a seed for insoluble tau aggregation. In a result, insoluble TauP301L-BiFC aggregates coaggregated with endogenous mouse tau accumulated in the brain, showing subsequently increased BiFC fluorescence from 9 months. Neuronal degeneration and cognitive deficits were observed from 12 months of age. TauP301L-BiFC mouse model demonstrated that methylene blue reduced the amount of soluble tau oligomers in the brain, resulting in the prevention of cognitive impairments. We assure that TauP301L-BiFC mice are a bona-fide animal tool to monitor pathological tau oligomerization in AD and other tauopathies.